Replacing windows is one of the most common home-improvement projects in the UK, and one where the process, paperwork, and costs are widely misunderstood. Using a FENSA-registered installer simplifies the Building Regulations element considerably. Budget £300–£1,500 per window fully installed; a typical three-bed semi with 10–12 windows runs £4,000–£12,000 depending on style, material, and glazing specification.

Why Replace Windows?

The most common reasons:

  • Failed sealed units, condensation trapped between panes indicates the seal has broken; the unit must be replaced.
  • Single glazing, pre-1980s homes often retain single-glazed metal or timber frames; replacement dramatically reduces heat loss and noise.
  • Damaged frames, rotted timber, cracked uPVC, or corroded steel frames that can no longer be repaired economically.
  • Upgrading thermal performance, switching from older double glazing (U-value 2.0+ W/m²K) to modern low-e argon-filled units (U-value 1.2 W/m²K) reduces heating bills noticeably.
  • Security, older windows with single-point catches can be upgraded to multi-point locking as standard.

Building Regulations

Replacing a window in England is controlled building work under Part L (Conservation of Fuel and Power).1 The replacement unit must achieve:

  • Whole-window U-value of 1.4 W/m²K or better, or
  • Window energy rating (WER) of Band B or above (using the BFRC scale from E to A++).23

All modern double-glazed windows with low-emissivity coatings and argon fill comfortably meet this standard. Compliance must be formally notified to the local authority, but there are two ways to do this.4

FENSA and Competent Person Schemes

FENSA (Fenestration Self-Assessment Scheme) is the most widely used Competent Person Scheme for window and door replacement.5 A FENSA-registered installer can self-certify that their work meets Building Regulations and notify the local authority on your behalf.5 You receive a FENSA certificate by post once the installer has registered the job with FENSA, which they must do within 21 days of completion, this is the document mortgage lenders and conveyancers ask for when you sell the property.6

Certass is the main alternative scheme, widely used by independent double-glazing companies.7

If the installer is not registered with either scheme, the householder must apply for Building Control approval from the local authority (typically £300–£500 for residential window work) before work starts.

What to check before appointing

  • Verify FENSA registration at fensa.org.uk using the company name or registration number.
  • Ask to see evidence of their current registration certificate, not just a logo on their van.
  • A legitimate registered installer will automatically include the FENSA notification in their standard service; you should not need to ask or pay extra.

The Replacement Process Step by Step

1. Survey

A pre-installation survey (usually free) checks:

  • Opening sizes and whether existing frames are structural (rare in modern housing but common in older properties with steel or timber structural frames).
  • Condition of lintels, reveals, and sills.
  • Any planning constraints (conservation area, listed building, or Article 4 Direction removing permitted development rights).

2. Specification and Quote

The written quote should itemise:

  • Frame material, colour, and profile specification.
  • Glazing unit specification (thickness, coating, gas fill, U-value or WER).
  • Hardware (handle type, trickle vent provision, hinge specification).
  • Whether installation includes making good (internal plaster, external mastic, repainting reveals).
  • FENSA or Certass registration (confirm this is included).
  • Waste disposal.

3. Order and Manufacturing Lead Time

uPVC and aluminium windows are made to measure. Standard lead times:

  • uPVC: 2–4 weeks from order.
  • Aluminium: 4–8 weeks.
  • Timber: 6–14 weeks (bespoke joinery takes longer).

4. Installation Day

A standard two-person team can replace 8–12 casement windows in a day. The process per window:

  1. Remove existing frame by cutting fixings and prising out the frame.
  2. Clean and measure the reveal; check lintel condition.
  3. Fix new frame into reveal using frame fixings at maximum 600 mm centres.8
  4. Level, plumb, and square the frame before tightening fixings.
  5. Hang sash or fit glazed unit.
  6. Seal externally with low-modulus silicone mastic; fill internal gaps with expanding foam and make good with plaster or render as agreed.
  7. Fit handles, trickle vents, and any restrictors (Part K calls for guarding at least 800 mm high at opening windows in a house where the drop outside is more than 600 mm).9

5. Post-Installation

  • Installer registers the job with FENSA within 21 days of completion; FENSA then posts the certificate to you (allow about two weeks).
  • Retain the certificate with your property documents.
  • Check all seals, handles, and opening actions before the installer leaves.

Cost Breakdown

Window typeuPVC (supply & fit)Aluminium (supply & fit)Timber (supply & fit)
Small casement (up to 0.5 m²)£300–£550£500–£900£700–£1,400
Standard casement (0.5–1.0 m²)£400–£800£700–£1,300£900–£1,800
Large casement / French casement£600–£1,100£1,000–£2,000£1,200–£2,500
Sash window (standard)£600–£1,200£900–£1,800£1,500–£3,500
Bay window (3-section)£1,200–£3,500£2,000–£5,000£3,000–£7,000+
Fixed/picture window£200–£500£350–£700£400–£900

Prices include standard installation and FENSA certification. Significant lintel repairs, structural reveals, or listed building consent work are additional.

Whole-house estimates (three-bed semi, 10–12 windows):

  • uPVC double-glazed: £4,000–£9,000
  • Aluminium double-glazed: £7,000–£15,000
  • Timber (softwood): £9,000–£18,000

Glazing Options

Double glazing remains the standard. A modern 4/16Ar/4 unit with low-e coating achieves a centre-pane U-value of approximately 1.1 W/m²K and a whole-window U-value (with uPVC frame) of around 1.2–1.4 W/m²K.

Triple glazing (4/14/4/14/4 with argon or krypton fill, double low-e coating) achieves whole-window U-values of 0.7–1.0 W/m²K. The additional cost is typically £80–£200 per unit over equivalent double glazing. The payback period is longest in milder UK climates; triple glazing is most justified for:

  • North or east-facing rooms in England above the Midlands.
  • Exposed coastal or upland locations.
  • Homes targeting EPC A or near-Passivhaus performance.

Acoustic glazing uses asymmetric pane thicknesses (e.g. 6/12/10) or laminated glass to improve sound reduction. Relevant for properties within 50 m of a busy road (>60 dB Lden) or under a flight path.

Conservation Areas and Listed Buildings

If your property is in a conservation area, you may need planning permission to replace windows if:

  • The existing windows are original single-glazed timber and you intend to change the material or profile.
  • An Article 4 Direction has removed permitted development rights (common in terraced conservation areas).10

Listed buildings require Listed Building Consent for any alteration.11 Replacement windows must normally match the original style, material, and profile, usually meaning secondary glazing (a second frame fitted inside the existing single-glazed window) rather than replacement, or a specialist slim-profile double-glazed timber unit manufactured to match the original.

Common Mistakes to Avoid

  • Accepting verbal FENSA assurance without checking, always verify registration online before appointing.
  • Ignoring lintel condition, a cracked or undersized lintel should be repaired before new frames go in; flagging this in the survey saves arguments later.
  • Forgetting trickle vents, Building Regulations (Part F) require background ventilation in habitable rooms; most replacement windows include a trickle vent in the head or frame; make sure your specification confirms this.12
  • Choosing the cheapest quote without checking the glazing spec, a quote that doesn’t state the U-value or WER may be using sub-specification units.
  • Skipping the snagging walk-round, check every window opens, closes, and locks before the installer leaves site.
References (12)

Figures in this note were checked against the sources below on 7 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongPlanning Portal, guidance.RN-33F8DPThe Planning Portal on building regulations for doors and windows: since 1 April 2002 the regulations have applied to all replacement glazing, covering thermal performance, safety glazing, means of escape and ventilation. That 2002 date is the one that matters at sale, because it is when a FENSA certificate or a building regulations notice started being expected for replacement windows.
  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. 3technicalstrongBritish Fenestration Rating Council, a page from the body itself.RN-XMDL91The BFRC describes itself as the UK's most trusted and widely recognised rating organisation for verifying the energy performance of windows and doors. Its Window Energy Rating runs on a scale from A++ down to E and combines three measured factors rather than one: solar gain, the heat travelling in through a window, counted positive; thermal transmittance or U-value, the heat loss out of a window, counted negative; and air leakage, the uncontrolled air flow out of a window that causes heat loss, also negative. BFRC makes the point that a rating is more comprehensive proof than a single U-value. This matters because Approved Document L Table 4.2 accepts either a 1.4 W/(m2K) U-value or Window Energy Rating Band B for replacement windows, so a reader can meet the standard by either route.
  4. 4technicalvery strongGOV.UK, guidance.RN-81EIUNThe government's own list of alterations that need building regulations approval, and the page to cite when a reader asks whether a bathroom or kitchen refit is notifiable. Works listed as needing approval include installing a bathroom that will involve plumbing, changing electrics near a bath or shower, replacing a fuse box and connected electrics, replacing windows and doors, replacing roof coverings, installing or replacing a heating system, adding radiators to an existing heating system, and installing fixed air conditioning. Emergency repairs to a boiler or heating system do not need approval in advance, though specific rules then apply. Approval may be obtained from a local authority building control department, a private registered building control approver, or, for higher-risk residential buildings, the Building Safety Regulator. The page states the practical consequence bluntly: without approval you will not have the certificates of compliance you may need when you want to sell your home. Note that a like-for-like bathroom refit with no new plumbing, no drainage change and no work in the electrical zones is not on the list.
  5. 5technicalstrongFENSA.RN-P9PNJKWhat FENSA is: a government-authorised competent person scheme that monitors building regulation compliance for replacement windows and doors, letting a registered installer self-certify instead of going to building control. This page is the basis for saying that a FENSA certificate is evidence of building regulations compliance rather than a guarantee of workmanship, which is the distinction most often got wrong.
  6. 6technicalstrongFENSA.RN-L9Z8STFENSA's rule that a member must register a completed window or door installation within 21 days. That registration is what produces the certificate a homeowner is given and what notifies building control, so the 21 days is the deadline behind a missing certificate. Relevant at sale, when a conveyancer asks for the FENSA certificate for any replacement windows since April 2002.
  7. 7technicalvery strongMinistry of Housing, Communities and Local Government.RN-SAMO3MThe official list of competent person schemes whose members can self-certify certain building work instead of getting building regulations approval, published 4 April 2013 and last updated 12 March 2018. Named schemes with contact numbers include ATTMA, APHC, Assure Certification, Blue Flame Certification, the British Board of Agrement, BESCA, Capita Gas Registration and Ancillary Services trading as Gas Safe Register, CIGA, Certass, Certsure LLP trading as NICEIC, the Elmhurst Airtightness Scheme, Fensa Limited, HETAS Limited, NAPIT Registration Limited, the National Federation of Roofing Contractors trading as CompetentRoofer, and OFTEC. Two footnotes matter: the Gas Safe Register is not technically a competent person scheme although in practice its installers carry the same building regulations responsibilities, and Benchmark Certification Limited ceased to operate as a competent person scheme on 10 February 2017, becoming part of NAPIT Registration Limited that day. Installers joining an electrical scheme are added to the single electrical competent person register.
  8. 8technicalgoodGlass and Glazing Federation, guidance.RN-J76S9IGlass and Glazing Federation good practice guide 20.3, April 2016, on installing replacement windows and doors. Covers survey, removal of the old frame, fixing centres and packing, sealing and weathering, and making good, which together are what separates a proper replacement from a frame pushed into a hole and mastic run round it. The trade body's own standard for its members.
  9. 9technicalvery strongMinistry of Housing, Communities and Local Government.RN-XH6E39The standard that decides whether a loft ladder is legal and when a raised deck needs a balustrade. Published 3 January 2013, it covers protection from falling, collision and impact including stairs, ladders, ramps, guarding and vehicle barriers. Paragraph 1.31 states that retractable ladders must not be used as a means of escape. Paragraph 1.32 permits a fixed ladder, with fixed handrails on both sides, only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a compliant stair, which is the rule that catches most loft ladder installations. Paragraph 1.29 restricts alternating tread stairs to loft conversions. For a private stair the maximum pitch is 42 degrees (Table 1.1 note 1), the normal relationship between rise and going is twice the rise plus the going between 550mm and 700mm, and for dwellings external tapered steps forming part of the building need a going of at least 280mm. Paragraph 3.1 requires guarding wherever it is reasonably necessary for safety at the edge of a floor, gallery, balcony, roof, light well, basement or similar sunken area. Paragraph 3.2 requires guarding to be at least the height in Diagram 3.1, allows any wall, parapet or balustrade to serve as guarding, and requires it to resist the loads in BS EN 1991-1-1 with its UK National Annex and PD 6688-1-1, with BS 6180 for barrier and infill panel design. Handrails should be positioned 900mm to 1000mm from the pitch line or floor.
  10. 10technicalvery strongPlanning Portal, guidance.RN-LIPHMWThe Planning Portal on planning permission for doors and windows: usually not needed for repairs, maintenance and minor improvements including replacing windows like for like, with the exceptions that catch people, listed buildings, conservation areas, flats, and where permitted development rights have been removed.
  11. 11technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-XN22L9The provision that decides when listed building consent is actually needed, and the answer to the common overstatement that any alteration to a listed building requires consent. Section 7(1): 'no person shall execute or cause to be executed any works for the demolition of a listed building or for its alteration or extension in any manner which would affect its character as a building of special architectural or historic interest, unless the works are authorised under section 8.' The trigger is therefore effect on character as a building of special architectural or historic interest, not alteration as such, so work with no effect on that character falls outside the restriction. Section 7(2) makes subsection (1) subject to section 33(1) of the Planning Act 2008, which excludes the requirement for other consents where development consent is required. Authorisation itself comes from section 8.
  12. 12technicalvery strongMinistry of Housing, Communities and Local Government.RN-QD1SFFTable 1.1 sets minimum intermittent extract rates: kitchen with a cooker hood extracting outside 30 l/s, kitchen without one 60 l/s, utility room 30 l/s, bathroom 15 l/s, sanitary accommodation 6 l/s. Extract terminals and fans other than cooker hoods should be as high as practicable and no more than 400mm below the ceiling; a cooker hood should sit between 650mm and 750mm above the hob if the manufacturer gives no figure. Table 1.7 requires background ventilators of at least 8000mm2 equivalent area in habitable rooms and kitchens for a dwelling, 10,000mm2 for a single-storey dwelling. Background ventilators should be at least 1700mm above floor level and at least 500mm from a fan in the same room. On replacement windows, paragraph 3.14 says new windows must carry background ventilators no smaller than the originals, and paragraph 3.15 gives the fallback minimums where the old size is unknown: 8000mm2 habitable rooms, 8000mm2 kitchen, 4000mm2 bathroom. Paragraph 3.32 confirms that replacing an extractor fan or cooker hood on existing cabling is not notifiable.

  1. Building Regulations - Doors and windows - Planning Portal, Planning Portal, accessed 7 September 2026. planningportal.co.uk

  2. Approved Document L: Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  3. Energy Ratings, BFRC (British Fenestration Rating Council), accessed 7 September 2026. bfrc.org

  4. Building regulations approval: When you need approval, GOV.UK, accessed 7 September 2026. gov.uk

  5. About FENSA, FENSA, accessed 7 September 2026. fensa.org.uk

  6. Register your work in 21 days, FENSA, accessed 7 September 2026. fensa.org.uk

  7. Competent person scheme - current schemes and how schemes are authorised, GOV.UK, accessed 7 September 2026. gov.uk

  8. A Guide to Good Practice: Installation of replacement windows and doors (GGF 20.3, April 2016), Glass and Glazing Federation, accessed 7 September 2026. ggf.org.uk

  9. Approved Document K: Protection from falling, collision and impact, 2013 edition, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  10. Planning Permission - Doors and windows - Planning Portal, Planning Portal, accessed 7 September 2026. planningportal.co.uk

  11. Planning (Listed Buildings and Conservation Areas) Act 1990, section 7: Restriction on works affecting listed buildings, legislation.gov.uk, accessed 7 September 2026. legislation.gov.uk

  12. Approved Document F: Ventilation, Volume 1: Dwellings, 2021 edition, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk