Green roof types and costs
Green roofs are classified by substrate depth and planting intensity. Extensive green roofs (sedum blanket systems, 60–100mm substrate depth) are the most affordable and widely installed type: £100–£180/m² installed on a flat roof extension. The sedum mat is pre-grown on a jute backing, supplied in rolls and installed over a drainage mat and root barrier. Total build-up adds 60–100kg/m² (dry) to the roof structure. Semi-intensive green roofs (mixed planting with grasses, herbs and low shrubs, 100–150mm substrate): £150–£250/m² — greater visual interest but requires slightly more maintenance than pure sedum. Intensive green roofs (planted roof garden with true soil depth of 200mm or more, capable of supporting shrubs and small trees): £300–£600/m² — a full horticultural system that requires structural loading design of 250–500kg/m² (wet), waterproofing and drainage engineered for long-term root resistance. Biodiverse brown roofs (crushed aggregate, rubble and sparse sedum to maximise invertebrate habitat): £80–£150/m² — the preferred specification for biodiversity net gain (BNG) credits in London planning applications. Blue-green roofs (water retention layer combined with planting for sustainable urban drainage credit): £180–£300/m² — increasingly specified on London developments to comply with SUDS requirements.
Green roofs and London planning policy
The planning case for green roofs in London has strengthened substantially in recent years. The London Plan Policy G5 (Urban Greening) requires new development to incorporate urban greening measures, with the Urban Greening Factor (UGF) tool used to assess performance. At borough level, Hackney, Islington, Camden, Tower Hamlets and Lewisham all have supplementary planning policies encouraging or requiring green roofs on new and replacement roof surfaces. The Environment Act 2021 introduced mandatory biodiversity net gain (BNG) — a requirement for all planning applications to deliver a 10% net gain in biodiversity. For a residential extension, the BNG requirement is typically met by a combination of a biodiverse green roof (brown roof specification with rubble and sparse sedum), habitat enhancements in the garden, and swift brick installation in external walls. A properly specified biodiverse flat roof on an extension delivers approximately 0.02–0.04 biodiversity units per m², which can contribute meaningfully to a BNG calculation. Planning officers in Hackney, Islington and Lewisham have specifically requested green roofs as conditions of rear extension approvals. Builderr includes a BNG assessment and green roof specification as part of all planning applications.
Technical requirements for green roofs
A green roof build-up on a flat extension consists of several distinct functional layers. From the structural deck up: the primary waterproofing membrane (EPDM, GRP or single-ply TPO — all are root-compatible without an additional root barrier when using a modern EPDM system; older bituminous felts require a separate root-barrier layer); a root protection layer (typically a 1mm HDPE sheet for older membrane systems); a drainage and water retention mat (25–50mm deep polyethylene drainage cells with fleece filter membrane above); engineered growing substrate (not topsoil — a lightweight aggregate:organic matter:grit mix designed to drain freely and resist compaction); and the planting layer (sedum mat, plug plants or seed mix). The critical design variable is structural loading. A typical sedum roof adds 60–150kg/m² depending on substrate depth and saturation. The extension's structural deck and timber or steel beams must be designed for this additional imposed load. Builderr's structural engineer reviews the loading capacity of the existing structure for all green roof projects and designs the joists or beam size accordingly if they are part of a new extension.
Maintenance of a green roof in London
One of the key selling points of extensive sedum roofs in London is their very low maintenance requirement — lower than any alternative planted or turfed surface. Sedum is a drought-tolerant succulent that is specifically adapted to the thin substrate and temperature extremes of a roof environment. In London's climate, sedum roofs do not require any irrigation — summer heat causes temporary browning and die-back of the sedum, but the plant recovers rapidly when autumn rain resumes. Annual maintenance visits (£80–£150 per visit) are recommended to remove any weed seedlings (buddleia and willowherb are the most common invasive species on London green roofs), check drainage outlets are clear of debris, and inspect the membrane at visible edge details. Semi-intensive roofs require two annual visits and occasional supplementary watering in extended dry spells. Intensive roof gardens require regular horticultural maintenance equivalent to a conventional garden, including watering, pruning, feeding and pest management. Sedum and biodiverse roofs should not be walked on except for maintenance — install a maintenance walkway strip if regular access is needed. Builderr provides an optional annual maintenance package for all green roof installations.
