The short answer
Yes, a sedum green roof can go on a house extension, provided the roof structure is designed to carry the extra weight of a wet growing layer and the waterproofing underneath is sound. The membrane must be fully watertight before anything is planted, because reaching a leak later means lifting the plants. Upkeep is light but never zero.
What a sedum roof is made of
Sedum is a low, fleshy plant that copes with thin soil and long dry spells. A sedum roof is built in layers. At the bottom sit the deck and the insulation, then the waterproof membrane. Above that goes a root barrier, a drainage layer that lets surplus water leave, a filter fabric and finally a thin growing medium planted with sedum, either as pre-grown mats or as cuttings and plugs scattered across the surface.
That sounds like a lot of extra work on top of an ordinary flat roof, and it is. The finished build-up is thicker, heavier and dearer than a bare membrane, so it is a choice made for looks, wildlife or a planning requirement, not for simplicity.
The plain waterproof coverings beneath it are covered elsewhere on this site, including single ply membranes for houses, which is one of the sheet types that suits a planted roof.
Weight and the structure underneath
Weight is the first question. The answer varies with the system, the depth of the growing layer and how wet it is. A saturated substrate is far heavier than a dry one, so the roof must be designed for the wet figure, plus snow, plus anyone who steps up for maintenance. We do not quote a figure here, because the supplier's technical sheet for the exact system you pick is the number your designer should use.
The Planning Portal warns that a much heavier replacement covering may mean modifying or strengthening the roof before work begins. It also says Building Regulations approval is needed once the weight rises by 15 per cent or more, and a planted roof will almost always cross that line, so expect your building control body to be involved from the start.
The load path matters as well as the total. Joists, bearing walls and any steel beam over a wide opening all have to cope, and a structural engineer is the right person to say so in writing. We would rather you paid for that opinion early than discovered a deflecting joist after the plants are down.
Waterproofing, falls and drainage
The membrane is the part you never want to see again, so it has to be right first time. Single ply sheet, GRP and lead can all sit beneath a green roof, but roots and constant moisture put more strain on joints and edges than a bare roof ever would. Many roofers will want the membrane checked for leaks before the layers go on top.
Falls still matter. A green roof holds some rainwater, but the structure should still shed water towards the outlets: BS 6229 asks for a design fall of 1:40 to guarantee a finished 1:80, as our page on flat roof ponding explains. Sedum dislikes sitting in standing water as much as the membrane does.
The edges need a clear gravel strip or a metal trim so that soil cannot block the outlets or spill over the fascia. Our notes on flat roof edge trims show what a tidy perimeter involves, and why the detail at the drip edge deserves as much care as the sheet itself.
Rules, permission and insulation
A green roof on an extension is not exempt from the usual checks. Part L treats a new flat roof covering as renovation of the thermal element, so insulation normally has to be brought up to date, and the thicker build-up needs checking against door thresholds and window cills. The wider picture is in our page on flat roof Building Regulations.
Planning is a separate question. A new extension can carry conditions of its own, and a council will sometimes ask for a planted roof, so ask the planning officers where you live, which is Swindon Borough Council for addresses inside the borough, before you design around one. Listed status and conservation area boundaries deserve a separate question. The plants themselves change none of these rules, although the extra weight and the deeper roof edge sometimes do.
Looking after it, and what we would advise
Sedum is low-maintenance rather than no-maintenance. Expect to check the outlets for blockage, pull out weeds and seedlings blown in from nearby trees, and look for bare patches that need replanting. The plants can brown in a dry summer and recover afterwards. Feeding and watering are generally light, depending on the system, and the supplier will give you a schedule to follow.
Cost is hard to generalise, so a planted roof on a house is priced on survey of the structure. What drives it is the area, the system chosen, the strength of the structure below and how easily materials can be lifted to the roof, which is why the quote follows a survey instead of a guess. Expect the total to sit well above a bare membrane of the same size.
Our advice is practical. If you want a green roof, mention it at the design stage so the structure, falls and membrane are specified for it. If you simply like the idea, a good plain membrane may serve the extension better and cost far less. Whichever you choose, the visit gives you pictures of your own roof, a written quote comes next and completed work is covered by our workmanship guarantee of ten years, in writing. Start with a phone call on 01793 626096, a WhatsApp message or the contact form. The full range is under flat roofing.