Green roofs, also called living roofs, cover a roof surface with a growing medium and plants, turning an otherwise dead thermal plane into a functioning ecosystem. In UK residential construction they are most commonly seen on single-storey extensions, garage roofs, and garden rooms, though they increasingly appear on new-build housing and flats where planners require biodiversity net gain (BNG) contributions.

They are not merely decorative. A well-built green roof extends the waterproof membrane lifespan (by protecting it from UV and thermal cycling), holds back rainwater, provides modest insulation, and, in urban areas, contributes to local biodiversity. The GRO Green Roof Code puts average annual runoff at 50% of rainfall for a green roof with 50 mm of substrate and 45% with 100 mm, against 81% for a standard roof.1 They can also help a scheme meet local plan greening policies, which in London and some other urban authorities expect green roofs on larger developments.

Types of Green Roof

The core distinction is between extensive and intensive systems, determined primarily by substrate depth and planting ambition.

TypeSubstrate depthWeight (saturated)Typical plantsCost (installed, per m²)
Extensive (sedum mat)60–100 mm80–150 kg/m²Sedums, mosses, herbs£60–£100
Extensive (wildflower/meadow)100–150 mm110–200 kg/m²Grasses, wildflowers£80–£130
Semi-intensive150–250 mm180–300 kg/m²Perennials, shrubs£100–£160
Intensive250–600 mm+300–700+ kg/m²Shrubs, small trees£150–£250+
Biodiverse/brown roof80–200 mm120–250 kg/m²Self-colonising, rubble substrate£65–£120

The per-m² figures are for the green roof layers on a sound, waterproof deck; a new root-resistant membrane and insulation beneath them add to the cost, as the 30 m² example below shows.

The GRO Green Roof Code recommends laying a sedum blanket on at least 60 mm of substrate, giving about 80 mm in total.1 A wildflower blanket should be laid on at least 100 mm.1

Most domestic projects use extensive sedum systems, pre-grown sedum mats or plug-planted trays installed on a purpose-built drainage and filter layer. These are the lightest and lowest-maintenance option.

Build-Up Layers

A green roof is a system, not just soil on a membrane. From structural deck upwards:

  1. Structural deck, timber joists with T&G decking (checked for adequate load capacity), or concrete/metal deck.
  2. Vapour control layer (VCL), foil-faced or reinforced, on the warm side of any insulation, taped at laps.
  3. Insulation, usually PIR boards (60–120 mm) or EPS depending on target U-value.
  4. Waterproof membrane, the most critical layer. Root-resistant hot melt asphalt, two-layer SBS-modified bitumen felt, or single-ply (TPO, PVC-P, or EPDM). All must be certified root-resistant, for example to BS EN 13948 or the German FLL test.12
  5. Root barrier, often a separate HDPE sheet if the membrane is not inherently root-resistant.
  6. Drainage layer, typically dimpled HDPE drainage cells (20–50 mm depth); retains some water for plant use during dry spells.
  7. Filter fleece, geotextile (150–200 g/m²) preventing fine particles washing into drainage.
  8. Growing medium, mineral-based (crushed brick, expanded clay, lava) blended substrate, not garden soil (which compacts and becomes waterlogged).
  9. Plants, sedum mat, plug plants, or seed mix depending on system type.

Pebble fire breaks at least 300 mm wide, increased to 500 mm next to doors, windows and opening rooflights, are required around all perimeters and penetrations such as rooflights, soil pipes and rainwater outlets; they also keep plants off upstands and allow membrane inspection.1

Structural Requirements

This is the gating question before any green roof project. Existing structures designed only for standard roofing loads may not accommodate even an extensive green roof at full saturation. The GRO Green Roof Code describes extensive roofs as typically weighing less than 250 kg/m² saturated (about 2.5 kN/m²), and a shallow sedum system is often 0.8–1.5 kN/m².1 Semi-intensive and intensive systems can easily reach 3–7 kN/m².

You will need a structural engineer to assess the roof structure, a typical fee of £300–£600 for a domestic garage or extension roof. Do not skip this. Overloading a roof structure is a serious safety hazard. Strengthening joists or adding mid-span support is possible but adds cost.

Drainage and Falls

Flat roofs are usually designed to a fall of 1:40 (about 1.4°) so that the finished roof still achieves at least 1:80 after deflection and site inaccuracies.3 Green roof systems can also go on pitched roofs up to about 30°, but steeper slopes need anti-shear measures to stop the substrate slipping.1 On truly flat decks, tapered insulation can create falls without altering the structure.

Rainwater outlets must be designed for the green roof’s reduced (but not eliminated) runoff. On roofs draining to public sewers, some local authorities require a runoff attenuation calculation (typically using the SuDS manual methodology) and may require an attenuating drain medium to achieve a specific discharge rate.

Planning and Building Regs

For most single-storey domestic applications (garage, extension, outbuilding), a green roof does not require planning permission under permitted development. However:

  • In conservation areas, listed buildings, or Article 4 directions, any material change to a roof’s appearance may need consent.
  • Green roofs on extensions that would otherwise be permitted development are fine, but the extension itself must still comply with PD limits.
  • Some local plans, particularly in London, encourage or require green roofs on new flat roofs, mostly on larger developments, so check with your LPA before designing.
  • Building Regs approval (Building Notice or Full Plans) is required for any structural alteration, new roof covering on an extension, or change to insulation/drainage. Putting a green roof on an existing flat roof is still controlled work: replacing the waterproof membrane on a flat roof counts as renovating a thermal element, so if more than half the roof is renovated the whole roof should be upgraded to the Table 4.3 U-value, and the added load needs checking against Part A.4

Biodiversity Net Gain

The Environment Act 2021 mandates a 10% biodiversity net gain. It applies to planning permissions granted on applications made on or after 12 February 2024 for major development, and the temporary exemption for non-major development expired on 2 April 2024.5 Green roofs can count towards the on-site score in the statutory biodiversity metric used to calculate BNG, but on a house the metric’s user guide says to record any green roof as ‘other green roof’ whatever its design.6 Note the name: the statutory biodiversity metric replaced Biodiversity Metric 4.0, and GOV.UK publishes guidance on transferring data from 4.0 to the statutory tool.7 Most home projects are outside it. Householder applications, the kind made for an extension, conservatory or loft conversion on your own house, are exempt from BNG, and so is work done under permitted development.8 GOV.UK also lists developments of 0.2 hectares or below among the exemptions.8 Where a project does require BNG, a green roof can be a practical contribution on a constrained urban site.

Maintenance

Extensive sedum roofs are genuinely low-maintenance, but not zero-maintenance:

  • Year 1: Water the roof while it establishes, typically for 6–8 weeks after installation depending on rainfall, and remove weeds by hand (herbicides risk membrane damage).1
  • Ongoing: One to three visits a year, usually including spring and autumn, to remove non-desirable plants, check outlets, clear perimeter gravel, and visually inspect membrane edges.1 Budget £150–£350 per visit for a professional inspection on a typical domestic roof.
  • Fertilising: Sedum roofs need very little. The GRO Green Roof Code does not recommend regular or high use of fertiliser on the low-nutrient growing medium, so feed sparingly if at all and avoid high-nitrogen products.1
  • Membrane inspection: A qualified roofer should carry out a full inspection every 5–10 years, lifting any loose perimeter edging to check flashings.

Cost Summary

For a 30 m² garage roof in 2026, a complete extensive sedum system including structural check, tapered insulation, root-resistant membrane, drainage layer, substrate, and sedum mat would typically cost:

  • Materials and installation: £2,400–£4,200
  • Structural assessment: £300–£600
  • Planning/Building Notice (if needed): £548 (householder planning application fee, England from 1 April 2026)910

Total project budget: £2,700–£5,350 for a 30 m² roof, before any structural strengthening works.

Building control charges are extra: each local authority fixes its own under a charging scheme, so check your council’s fee for a building notice or full plans application.11

References (11)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalfairGreen Roof Organisation, guidance.RN-26W44OThe UK green roof industry code of practice. Its Table 1 gives average annual rainfall run-off by roof type: 81% for a standard roof, 50% and 45% for green roofs with 50 mm and 100 mm of substrate.
  2. 2technicalstrongBSI, guidance.RN-7D8OFRThe BSI catalogue entry for BS EN 13948, which confirms the standard exists and is the root penetration test for bitumen, plastic and rubber roof waterproofing sheets.
  3. 3technicalfairLiquid Roofing and Waterproofing Association, guidance.RN-Q076GKA trade association guidance note on flat roof falls. It cites BS 6229's 1:80 minimum finished fall and explains that 1:40 or 1:60 design falls are used to allow for deflection and site inaccuracies.
  4. 4technicalvery 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.
  5. 5technicalvery strongGOV.UK, guidance.RN-HXRMK1The government's guidance on mandatory biodiversity net gain. It gives the 10 per cent objective and the dates from which the duty applies to major and non-major development.
  6. 6technicalvery strongDepartment for Environment, Food and Rural Affairs, guidance.RN-UUNLEJDefra's user guide to the statutory biodiversity metric. It says green roofs on dwellinghouses count towards the on-site score but must be recorded as 'other green roof' regardless of design.
  7. 7technicalvery strongDepartment for Environment, Food and Rural Affairs, guidance.RN-VJ3HJUThe GOV.UK page for the statutory biodiversity metric. It publishes the tool and guides, including guidance on moving data from the earlier Biodiversity Metric 4.0.
  8. 8technicalvery strongDepartment for Environment, Food and Rural Affairs, guidance.RN-IM04HJGOV.UK guidance listing the developments exempt from mandatory biodiversity net gain in England: householder applications (extensions, conservatories, loft conversions), developments of 0.2 hectares or below, de minimis developments, temporary developments, self-build and custom build, and others.
  9. 9technicalvery strongMinistry of Housing, Communities and Local Government (GOV.UK), a government page.RN-JM9I1NThe schedule of planning application fees in England from 1 April 2026. Fees rise annually with the September CPI under the 2023 amendment regulations; September 2025 CPI was 3.8%, so every fee from April 2025 rose by that and is charged from 1 April 2026. The table lists the new amount for each application type, which is the statutory figure a council must charge rather than a guide price.
  10. 10costvery strongPlanning Portal, guidance.RN-M2KUCTA price, true at 2026-04. The Planning Portal's consolidated guide to planning application fees in England from 1 April 2026, built from the 2012 fees regulations as amended and MHCLG's annual indexation. More usable than the regulations themselves because it puts each application type next to its current fee and its definition. The statutory amounts are in the regulations; this is the readable index to them.
  11. 11technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-JUSY4RRegulation 3 of the Building (Local Authority Charges) Regulations 2010 authorises each local authority to fix its own building regulations charges through a charging scheme.

  1. The GRO Green Roof Code: Green Roof Code of Best Practice, Anniversary Edition 2021, Green Roof Organisation, accessed 30 September 2026. greenrooforganisation.org

  2. BS EN 13948:2007, Flexible sheets for waterproofing. Bitumen, plastic and rubber sheets for roof waterproofing. Determination of resistance to root penetration, BSI, accessed 30 September 2026. knowledge.bsigroup.com

  3. Guidance Note No. 7: Specifier Guidance for Flat Roof Falls, Liquid Roofing and Waterproofing Association, accessed 30 September 2026. lrwa.org.uk

  4. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 30 September 2026. assets.publishing.service.gov.uk

  5. Biodiversity net gain, GOV.UK, accessed 13 September 2026. gov.uk

  6. The Statutory Biodiversity Metric: User Guide, June 2026, Department for Environment, Food and Rural Affairs, accessed 30 September 2026. assets.publishing.service.gov.uk

  7. Statutory biodiversity metric tools and guides, Department for Environment, Food and Rural Affairs, accessed 30 September 2026. gov.uk

  8. Biodiversity net gain: exempt developments, Department for Environment, Food and Rural Affairs, accessed 29 September 2026. gov.uk

  9. Fees for planning applications in England from 1 April 2026, Ministry of Housing, Communities and Local Government, accessed 7 September 2026. assets.publishing.service.gov.uk

  10. A guide to fees for Planning Applications in England, 1 April 2026, V2026, Planning Portal, accessed 15 September 2026. Priced at 2026-04, so no calibration is needed. ecab.planningportal.co.uk

  11. The Building (Local Authority Charges) Regulations 2010, regulation 3, legislation.gov.uk, accessed 2 October 2026. legislation.gov.uk