The short answer
Often yes, if the roof structure can carry the added load and the waterproofing beneath the planting is sound. Soil, plants and retained water weigh far more than a normal covering. A covering 15 per cent or more heavier needs Building Regulations approval and a structural check, according to the Planning Portal. A survey and an engineer's view come before any order for plants.
Weight, and who checks it
A green roof replaces a light waterproof covering with layers: a drainage sheet, a growing medium, plants, and the water they hold after rain. Even a thin planted layer weighs more than felt or rubber, and a deeper one weighs a good deal more.
The rules treat added weight as a structural matter. Approved Document A treats a load increase of more than 15 per cent as a significant change in roof loading. The Planning Portal sets the line at a new covering that adds 15 per cent or more to the weight, which brings Building Regulations approval into play, and it advises seeking the view of a structural engineer or surveyor first.
The question is therefore whether the extension's joists, beams and supporting walls can take the new load, including water-saturated soil and, in places, snow. That depends on spans, timber sizes and how the roof was built, and it cannot be judged from a photograph.
On a new extension, the answer is best settled at design stage so the structure is sized for the roof. On an existing flat roof, an engineer's check comes first. Our flat roofing page explains how a roof is surveyed.
Waterproofing under the planting
The membrane beneath a green roof matters more than usual, because a leak is harder to find and costly to reach. To get at the covering, the planting must be lifted or cut away.
Water also stays on the roof longer. A green roof is designed to hold and slow rain, so the membrane is wet more of the time than an open roof. It needs to be a tough product, laid with sound seams and upstands, and tested before planting goes on. Plant roots can attack some coverings, so a membrane made for green roofs is the one to choose.
The deck must be sound and the falls correct. BS 6229:2025 keeps a minimum finished fall of 1:80, and IKO advises designing the roof for 1:40. Our answer on the minimum fall on a flat roof explains it. Outlets need protection from soil and roots, and must stay reachable for inspection.
Lifespans give context for the decision. HomeBuilding gives EPDM 25 to 30 years and GRP 25 to 30, and Less gives 30 to 50 for EPDM and 25 to 40 for GRP. A green roof is only as durable as the membrane under it, so laying new waterproofing beneath it is wise.
Building Regulations and planning
Two sets of rules may apply. The first is the Building Regulations. Beyond the load question, replacing the waterproof membrane on an existing flat roof is renovation in the eyes of Approved Document L, so insulation has to be considered, and a green roof build-up can be combined with it. On a new extension, the roof is a new element with its own targets.
Your building control body can say what it needs, whether a full plans application, a building notice or a registered building control approver. Do not assume that a green roof is exempt because it is soft or small.
The second is planning. A new extension is a planning question in its own right, and permitted development has conditions. For an existing extension, re-roofing a house falls within Class C, which has conditions of its own. One of them caps at 0.15 metres how far an alteration may rise above the original roof plane, so ask your local planning authority whether a deep build-up affects it.
A listed house, a conservation area or an Article 4 direction can each bring extra consent into play. Put the question to the council's planners early, before any plants are ordered.
Maintenance, access and the practical questions
A green roof is not a fit-and-forget finish. Plants need water in dry spells, weeds need removing, and drainage needs checking. Leaves and silt collect in outlets just as on a bare roof, as our answer on a blocked flat roof outlet describes, but outlets under planting are harder to see.
Access matters. A roof people will walk on to tend plants needs a safe route, edge protection and a hatch or stair, and the weight of people and watering cans is one more load for the engineer to consider. Some coverings tolerate occasional foot traffic better than others, and our answer on walking on a flat roof explains the differences.
We survey the existing structure, record the roof, and set out whether a green roof looks workable, who needs to check what, and how the waterproofing would be handled. If an engineer is needed, that is the first step. Once the survey is done, the written price covers the agreed roofing work in one total, and the structural check is arranged with whoever carries it out.
