Planning permission and Building Regulations are two entirely separate legal requirements, and confusing them is one of the most common mistakes homeowners make at the start of an extension project. You can need one without the other, both, or neither, but for the vast majority of single- and two-storey extensions, Building Regulations approval is mandatory. This guide explains what the regulations actually cover and how to navigate the process.

What Building Regulations Cover (and Why They Exist)

Building Regulations set the minimum standards for the design and construction of buildings in England and Wales. They protect the health and safety of occupants, neighbouring properties, and future owners. Unlike planning permission (which is about whether something should be built), Building Regulations are about how it must be built.

The regulations are organised into Approved Documents, one for each subject area. The ones that typically apply to a house extension are:

PartSubjectWhat it means for your extension
AStructureFoundations, walls, beams, lintels, roof loads
BFire safetyEscape routes, fire doors, smoke alarms
CSite prep and moistureDamp-proof courses, subfloor ventilation
FVentilationTrickle vents, extract fans in kitchens/bathrooms
JCombustion appliancesIf you’re installing a stove or boiler
LConservation of fuel and powerInsulation U-values for walls, roof, floor, and glazing
MAccessibilityStep heights, door widths, sanitary facilities
PElectrical safetyNew circuits, consumer unit replacement, work in a special location

Part Q (security) does not apply to an extension. It covers new dwellings only, including a dwelling formed by a material change of use, so work to an existing house falls outside it.1 PAS 24:2022-certified doors and ground-floor windows are still worth specifying on an extension, but that is a choice rather than a Building Regulations requirement.

Do I Need Building Regulations for My Extension?

The main exemption is for qualifying conservatories (Regulation 9 and Schedule 2 of the Building Regulations 2010, with Approved Document L setting the matching test for the energy efficiency requirements). Five conditions have to be met: the extension is at ground level, its floor area does not exceed 30 m², the glazing complies with Part K, the wall, door or window separating it from the house has been retained or replaced, and the dwelling’s heating system is not extended into it.2 Remove any one of those and the exemption disappears. There is no 75% roof glazing threshold in the current rules, that test came from the superseded 2010 edition of Approved Document L1B.

Porches are exempt on the same terms, and note which way the door test runs: the door between the porch and the house has to be retained (or replaced), not removed.2 Certain detached outbuildings under 30 m² are also exempt, depending on proximity to boundaries and construction type.3

For everything else, a rear extension, side return infill, two-storey addition, above-garage conversion, Building Regulations apply.

How to Apply: Full Plans vs Building Notice

There are two main application routes:

Full Plans application: You submit detailed architectural drawings and specifications before work starts. The local authority (or registered building control approver) checks the drawings against all relevant Parts and issues a formal approval notice, typically within 5 weeks (or a longer period agreed in writing for complex schemes).4 Inspectors then visit the site at key stages. On completion, you receive a Completion Certificate. This route is usually the better choice for anything beyond a small, simple extension.

Building Notice: You notify the local authority of your intention to build, and at least two clear days must pass before you start.5 No drawings are submitted in advance, the inspector reviews the work on site at each stage and advises you of any issues. Faster to start, but if the inspector requires changes after foundations are poured, the cost of remediation falls on you. It is not available at all for work that Part H4 covers, building over or within 3 m of a sewer, because regulation 12 requires a full plans application for that.6 Two separate things are involved and neither is a section 80 notice: connecting a drain to a public sewer is a right exercised by notice under section 106 of the Water Industry Act 1991, and building over or close to a public sewer is a Part H4 matter settled by a build-over agreement with the water company.7

Charges are set locally by each council, but for a single-storey extension of up to 40 m² they typically come to £830–£1,122, rising to £1,105–£1,610 for an extension of 40 to 100 m². You can also use a private registered building control approver (the name for what used to be called an approved inspector; the firms have been registered with the Building Safety Regulator since 6 April 2024), fees are negotiated and often comparable to local authority charges.8

Foundations

The Building Control inspector will expect to see the foundation excavation before concrete is poured. The required depth depends on ground conditions and proximity to trees:

  • Strip foundation depth on clay should be at least 750 mm on low-shrinkage clay, 900 mm on medium-shrinkage clay and 1,000 mm on high-shrinkage clay, and more where there are trees nearby.9 On other ground Approved Document A sets a minimum of 450 mm to keep the footing clear of frost, and its Table 10 gives the minimum width of a strip footing for a given ground condition and load, not its depth.9
  • Near trees on clay: can exceed 2,500 mm to get below the zone of seasonal moisture variation, a structural engineer should advise
  • Width of strip foundation: commonly dug 450–600 mm wide in practice, but the minimum for your wall load and ground comes from Table 10 or from the engineer

If you are building close to existing foundations (within 1 m or at a lower level), the inspector may require an engineer’s report confirming the existing footings are adequate.

Structural Requirements

Part A governs the structural integrity of the extension itself and its connection to the existing building. Typical requirements:

  • Lintels over openings: end bearing of at least 150 mm each side, which Approved Document A allows to drop to 100 mm where the clear span is 1.2 m or less; specified by a structural engineer or from the maker’s span tables9
  • Cavity walls: 100 mm blockwork inner leaf (at least the compressive strength Approved Document A sets for the wall’s loading, 2.9 or 7.3 N/mm² depending on the case), 100 mm facing brick outer leaf, and enough cavity insulation to reach the Part L limit of 0.18 W/m²K for a new wall, which usually means a cavity wider than the traditional 100 mm or a higher-performance board2
  • Roof structure: flat roof joists are sized from span tables or an engineer’s calculations; for pitched roofs, rafters are typically sized from the TRADA span tables
  • Wall ties: stainless steel, at no fewer than 2.5 ties per square metre (in practice 900 mm horizontal and 450 mm vertical centres), with extra ties at no more than 300 mm vertical spacing within 225 mm of openings and edges9

If you are removing a section of the existing external wall to merge the extension with the house, the structural beam over that opening must be designed by a structural engineer and signed off by Building Control.

Insulation (Part L)

Approved Document L (2021 edition, in force in England from 15 June 2022) sets maximum U-values for new fabric in an existing home, and those are the figures an extension has to meet.2

ElementMaximum U-value (W/m²K)
External walls0.18
Floor0.18
Roof (flat or pitched)0.15
Windows and doors (whole unit)1.40
Rooflights2.2

These are the Table 4.2 limits for new fabric elements in existing dwellings, which Approved Document L applies to elements in extensions.2 The total area of windows, roof windows, rooflights and doors should also not exceed 25% of the extension’s floor area plus the area of any windows and doors the extension removes or covers; if you want more glazing than that, you have to show by an area-weighted U-value or a SAP calculation that the extension performs no worse than one built to the standard.2 In practice a standard 100 mm cavity with full-fill insulation often will not reach 0.18 W/m²K, so ask for a U-value calculation for the actual build-up rather than assuming it will pass.

Drainage and Waterproofing (Part C and H)

If the extension footprint covers any underground drainage runs, you will need to either divert them or cover them with an access chamber, building over or within 3 m of a public sewer needs a build-over agreement from the water company, which flows from Part H4 of the Building Regulations rather than from a section of the Water Industry Act; the section of that Act you will meet is 106, the right to connect a drain to a public sewer.107

Part C requires damp-proof courses at a minimum of 150 mm above external finished ground level and a continuous DPC linked to the existing DPC in the house wall.11 Suspended timber floors need underfloor void ventilation, and the figure has two limbs with the larger governing: 1,500 mm² per metre run of external wall or 500 mm² per square metre of floor area, whichever is the greater. On a deep-plan floor the area limb is the one that bites, and quoting only the perimeter figure under-ventilates it. Solid floors need a damp-proof membrane below the insulation.11

Fire Safety (Part B)

A single-storey rear extension rarely creates new fire risks, but Part B requirements that typically apply include:

  • Smoke alarms: where the extension adds a new habitable room above or below the ground storey, or a ground-storey one without a final exit, Approved Document B asks for a Grade D2, Category LD3 system, with mains-powered alarms in the circulation spaces of the dwelling12
  • Fire doors where the extension creates a new inner room or where the escape route passes through the extension to the staircase
  • If the extension brings a boundary wall within 1 m of the boundary: maximum permitted unprotected area in that wall is very restricted, check with Building Control before designing large bifold doors in a side wall close to the fence

Electrical Work (Part P)

Only three things are notifiable in England: a new circuit, replacing the consumer unit, and an addition or alteration in a special location, which means within a room containing a bath or shower, from floor level up to 2.25 m and within 0.6 m of the bath or shower tray, or within 1.2 m of the shower head centre where there is no tray.13 An extension almost always involves at least one new circuit, so this bites. Notifiable work must either be done by a registered competent person (NICEIC, NAPIT or another scheme authorised by government) who self-certifies it, or notified to Building Control in advance so it can be inspected and tested. Everything else, including adding a socket to an existing circuit or moving one, is not notifiable.

Inspections and Sign-Off

A typical inspection schedule for a single-storey extension:

  1. Commencement, notify Building Control before work begins
  2. Foundations, before concrete is poured
  3. Oversite, DPC level, before floor is laid
  4. Drains, before backfilling
  5. Structural frame / roof (where applicable)
  6. Insulation / pre-plasterboard, so the inspector can check wall and roof insulation
  7. Completion, final inspection; inspector will check ventilation, fire detection, electrical certificate, and visible finishes

On satisfactory completion, the local authority issues a Completion Certificate (or the registered building control approver issues a Final Certificate). Keep this document safely, it is required when selling the property and is checked by solicitors during conveyancing. Without it, your solicitor may require you to take out indemnity insurance.

References (13)

Figures in this note were checked against the sources below on 30 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongMinistry of Housing, Communities and Local Government.RN-VGR6O6The security standard for doors and windows, ISBN 978 1 85946 601 8, in force from 5 March 2015 and last updated 22 November 2024. Requirement Q1 applies only to new dwellings, and to easily accessible doors and windows giving access into a dwelling from outside, into a building containing flats from outside, or into a flat from the common parts. Paragraph 1.2 requires secure doorsets tested to PAS 24:2012 or designed to Appendix B, with STS 201 Issue 5:2013, LPS 1175 Issue 7:2010 security rating 2, STS 202 Issue 3:2011 burglary rating 2 and LPS 2081 Issue 1:2015 security rating B accepted as equivalents. Paragraph 1.3 caps a letter plate aperture at 260mm x 40mm and points to DHF technical specification TS 008:2012. Paragraph 1.4 requires a door viewer on the main entrance door unless clear glass or an adjacent window serves the purpose, plus a door chain or limiter. Paragraph 2.2 requires ground floor, basement and other easily accessible windows and rooflights to be tested to PAS 24:2012, with STS 204 Issue 3:2012, LPS 1175 rating 1 and LPS 2081 rating A as equivalents, and paragraph 2.3 requires frames to be mechanically fixed to the structure. Appendix B specifies a doorset in solid or laminated timber to a minimum density.
  2. 2technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  3. 3technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Y7MVZKClass 7 is the conservatory and porch exemption every extension note leans on: the erection at ground level of an extension consisting of a conservatory, porch, covered yard or covered way, or a carport open on at least two sides, is exempt from the building regulations where the floor area does not exceed 30 square metres. Where a conservatory or porch is wholly or partly glazed, the glazing must still satisfy Part K4 and K5.1 to K5.4 and Part N of Schedule 1. Class 6 exempts small detached buildings up to 30 square metres single storey and up to 15 square metres otherwise, which is the garden room threshold.
  4. 4technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-A6L0BBRegulation 14A of the Building Regulations 2010, determination of applications for building control approval with full plans. The authority must grant approval unless the application fails regulation 14's requirements or the work would contravene the regulations. The provision that turns a full plans application into a decision the authority has to justify, which is the practical difference between full plans and a building notice.
  5. 5technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-6CCBA1Regulation 16(1) bars starting building work until notice of intention to start has been given to the relevant authority and at least two days have elapsed since the end of the day the notice was given. Regulation 16(3) lets the authority serve written notice requiring notification when a specified stage is reached, and may set a period before or after the work within which to notify and a period during which the work must not be covered up; under 16(3A) and 16(3B) a stage may only be specified if the authority intends to inspect it, based on its assessment of the risk of a breach. Regulation 16(3C) requires notice that work has commenced within five days of commencement, with a rejection notice possible within four weeks (16(3E)), deemed commencement if no rejection is given (16(3F)), and an appeal to the First-tier Tribunal within 21 relevant days (16(3H)) on grounds of error of fact, error of law, unreasonableness or failure to follow procedure. Regulation 16(4) requires notice within five days of completion.
  6. 6technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-37ODVXRegulation 12(1) applies to anyone intending to carry out building work, replace or renovate a thermal element, change a building's energy status or make a material change of use. Regulation 12(2) requires that person to give the relevant authority either a building notice under regulation 13 or an application for building control approval with full plans under regulation 14. Full plans are compulsory where the Regulatory Reform (Fire Safety) Order 2005 applies (12(3)), where the building fronts onto a private street (12(4)) and where paragraph H4 of Schedule 1 applies (12(5)). Regulation 12(6) exempts work done by a competent person listed in Schedule 3, work listed in Schedule 4, and work inspected by a Schedule 3A third party certifier appointed before work starts. Regulation 12(6A) still requires notification for Part P electrical work consisting of a new circuit, a consumer unit replacement, or any addition or alteration to existing circuits in a special location such as the zone around a bath or shower. Regulation 12(8) allows a building notice after the event for emergency repairs. Text is up to date to 7 September 2026.
  7. 7technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-PS0WRUThe statutory right that decides whether garden or driveway surface water can go to the sewer at all. Subsection (1) gives the owner or occupier of premises, and the owner of a private sewer, the right to have their drain or sewer communicate with the public sewer and to discharge foul water and surface water, subject to the section. Subsection (4) lets the sewerage undertaker refuse within 21 days of notice where the drain or sewer does not satisfy the standards reasonably required by the undertaker, or where the communication would be prejudicial to the undertaker's sewerage system. Subsection (2) prohibits connections that would allow discharge of liquid from a factory (other than domestic sewage or surface or storm water), any liquid whose discharge is prohibited by an enactment, foul water into a surface-water sewer or (without approval) surface water into a foul sewer, and any direct connection into a storm-water overflow sewer. The undertaker may require the lateral drain to vest under section 102. Disputes about the reasonableness of a refusal or requirement are determined by the Authority under section 30A.
  8. 8technicalvery strongPlanning Portal, guidance.RN-WV45MHPlanning Portal guidance on private building control. It explains that registered building control approvers replaced approved inspectors, with registration required by 6 April 2024.
  9. 9technicalvery strongMinistry of Housing, Communities and Local Government (GOV.UK).RN-Z6AXUWThe full Approved Document A PDF, which carries the deemed-to-satisfy tables a domestic project is checked against. Section 2E covers foundations of plain concrete: paragraph 2E3 gives the recommended minimum widths of strip foundations in Table 10, keyed to ground type and total load of loadbearing walling per linear metre. Table 10 sets, for rock (not inferior to sandstone, limestone or firm chalk, requiring at least a pneumatic or mechanically operated pick to excavate), a width equal to the width of the wall at every load. For medium dense gravel or sand (requires a pick to excavate, a 50mm square wooden peg hard to drive beyond 150mm) and for stiff clay or stiff sandy clay (can be indented slightly by the thumb), the minimum widths are 250mm at 20 kN per linear metre, 300mm at 30, 400mm at 40, 500mm at 50, 600mm at 60 and 650mm at 70. Firm clay and firm sandy clay (thumb makes an impression easily) start at 300mm. The document also covers minimum depth of strip foundations, wall thickness rules for residential buildings up to three storeys (paragraph 2C10 covers internal loadbearing walls in brickwork or blockwork), buttressing, lateral support by floors and at roof level, interruption of lateral support, wall cladding in Section 3, and Section 5 on reducing sensitivity to disproportionate collapse with the building consequence classes.
  10. 10technicalvery strongMinistry of Housing, Communities and Local Government.RN-SM6TRWThe statutory source for soakaway design and the discharge hierarchy every garden drainage note should state. Requirement H3(3) lists the order of priority for rainwater discharge: (a) an adequate soakaway or other adequate infiltration system, or where not reasonably practicable (b) a watercourse, or where not reasonably practicable (c) a sewer. Paragraph 3.25 bars infiltration devices within 5 metres of a building or road, in ground where the water table reaches the bottom of the device at any time of year, too close to other drainage fields or soakaways, or where contamination could pollute groundwater. Paragraph 3.26: soakaways serving less than 100 square metres are generally square or circular pits filled with rubble or lined with dry-jointed masonry or perforated ring units. Paragraph 3.27: design to a return period of once in ten years, and for small soakaways serving 25 square metres or less a design rainfall of 10mm in 5 minutes may be assumed as the worst case. Paragraph 3.28 requires percolation tests to derive the infiltration rate, and paragraph 3.30 sends larger soakaways to BS EN 752-4 or BRE Digest 365 Soakaway design. Paragraphs 2.10 to 2.12 cover pervious paving over a granular storage reservoir and warn against its use where sediment or oil spillage would block the pores. For foul drainage, paragraph 2.33 sets a minimum 75mm internal diameter, or 100mm for a WC, and Table 6 gives minimum gradients of 1 in 40 for 75mm and 100mm at peak flows under 1 litre per second.
  11. 11technicalvery strongMinistry of Housing, Communities and Local Government.RN-NPO8FJThe statutory figures behind a damp diagnosis, and the numbers to quote when a note explains why ground levels or a bridged damp proof course cause rising damp. Paragraph 5.2 requires that in an external wall the damp-proof course should be at least 150mm above the level of the adjoining ground (Diagram 8) unless part of the building protects the wall, and that in an external cavity wall the cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray provided (Diagram 9a). For suspended timber ground floors it requires a ventilated air space of at least 75mm from the ground covering to the underside of any wall-plates and at least 150mm to the underside of the suspended timber floor or insulation, with ventilation openings on two opposing external walls of not less than 1,500mm2 per metre run of external wall or 500mm2 per square metre of floor area, whichever is greater; for suspended concrete floors the clear ventilated space is at least 150mm with the same opening sizes. Section 2.39 onwards covers radon, referring to BRE Report BR 211 for protective measures. The 2013 edition was published on 3 September 2013 with changes in effect from 1 October 2013.
  12. 12technicalvery strongMinistry of Housing, Communities and Local Government.RN-J0UZDBApproved Document B Volume 1, fire safety in dwellings, 2019 edition with the 2020, 2022 and 2025 amendments and the forthcoming 2026 and 2029 changes collated in. Covers means of warning and escape, internal fire spread through linings and structure, external fire spread and access for the fire service. This is the document behind escape window sizes, protected stairways, the 4.5 metre rule and the fire doors a loft conversion needs.
  13. 13technicalvery strongMinistry of Housing, Communities and Local Government.RN-JYJ3I5The document that decides what electrical work a homeowner may legally do themselves, and the exact list of notifiable jobs. Regulation 12(6A), reproduced at paragraph 2.5, makes only three things notifiable: the installation of a new circuit, the replacement of a consumer unit, and any addition or alteration to existing circuits in a special location. Regulation 12(9) defines special location as, in a room containing a bath or shower, the zone extending vertically from finished floor level to 2.25 metres (or the shower head position if higher) and horizontally 600mm from the edge of the bath tub or shower tray. Paragraph 2.7 states that all other electrical installation work is not notifiable, namely additions and alterations to existing installations outside special locations, and replacements, repairs and maintenance anywhere. Paragraph 2.8 gives worked examples: a built-in cooker is not notifiable unless a new cooker circuit is needed, and connecting an electric gate or garage door to an existing isolator is not notifiable but running a new circuit from the consumer unit to that isolator is. Paragraph 2.2 confirms Part P reaches outdoor work including fixed garden lighting, pond pumps and outbuildings such as sheds, detached garages and domestic greenhouses. Notifiable work must be certified by self-certification by a registered competent person, third-party certification, or a building control body (paragraph 3.1), with the compliance certificate issued within 30 days (paragraph 3.4). Note that Approved Document P is a source of guidance rather than a ban: non-notifiable work still has to comply with BS 7671.

  1. Approved Document Q, Security, dwellings, 2015 edition, Ministry of Housing, Communities and Local Government, accessed 9 September 2026. assets.publishing.service.gov.uk

  2. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 10 September 2026. assets.publishing.service.gov.uk

  3. The Building Regulations 2010, Schedule 2: exempt buildings and work, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  4. The Building Regulations 2010, regulation 14A, legislation.gov.uk, accessed 7 September 2026. legislation.gov.uk

  5. The Building Regulations 2010, regulation 16: notice of commencement and completion of certain stages of work, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  6. The Building Regulations 2010, regulation 12: giving of a building notice or an application for building control approval, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  7. Water Industry Act 1991, section 106: Right to communicate with public sewers, The National Archives, accessed 14 September 2026. legislation.gov.uk

  8. Registered building control approver services, Planning Portal, accessed 30 September 2026. planningportal.co.uk

  9. Approved Document A, Structure, 2004 edition incorporating 2004, 2010 and 2013 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  10. Approved Document H, Drainage and waste disposal, 2015 edition, Ministry of Housing, Communities and Local Government, accessed 30 September 2026. assets.publishing.service.gov.uk

  11. Approved Document C, Site preparation and resistance to contaminants and moisture, 2004 edition incorporating 2010 and 2013 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  12. Approved Document B (fire safety) volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, Ministry of Housing, Communities and Local Government, accessed 30 September 2026. assets.publishing.service.gov.uk

  13. Approved Document P, Electrical safety, Dwellings, 2013 edition, Ministry of Housing, Communities and Local Government, accessed 9 September 2026. assets.publishing.service.gov.uk