What a flat roof extension actually costs to roof

When people ask what a flat roof extension costs, they usually mean the whole build. This guide covers only the part we do — the roof itself — so you can sanity check the roofing line on a builder's quote and see exactly what should be in it.

The short answer

For a standard single-storey rear extension in the TW and KT postcodes, budget roughly £110–£180 per square metre for a warm-deck EPDM or single-ply roof including firrings, insulation, membrane and trims. A 20m² extension roof typically lands between £2,400 and £3,600 before glazing, scaffold or parapet work.

What the roofing element of an extension includes

On a typical rear extension the roofing package is the structural deck (usually 18mm WBP ply or OSB3 laid over the joists), firring pieces to create the fall, a vapour control layer, rigid PIR insulation, the waterproof membrane, edge trims and drips, an outlet and downpipe connection, and the flashing or cover detail where the roof meets the house wall. If a builder's quote just says 'flat roof — £2,000', ask which of those eight items are included. In our experience the ones most often left out are the insulation upgrade and the abutment flashing, and those are the two that cause the arguments later.

Cost per square metre, honestly

Across Hampton, Hampton Hill, Twickenham and Teddington in 2026 we generally see warm-deck EPDM landing around £110–£150 per square metre fitted, single-ply (TPO or PVC) around £130–£170, and GRP fibreglass around £140–£180. Those figures assume a straightforward rectangle at single-storey height with reasonable access through a side return or garden. Anything that has to be craned, carried through a house or worked off a scaffold at first-floor level adds cost that has nothing to do with the membrane you choose.

Why the falls matter more than the material

Building Regulations expect a minimum finished fall of 1:80, and because timber sags and firrings are rarely perfect, competent roofers design to 1:60 or better. Standing water is not an instant leak, but it holds grit, accelerates UV damage, freezes in winter and shows every deflection in the deck. Getting the falls right is a design and carpentry cost, not a membrane cost, and it is the single most reliable predictor of whether an extension roof reaches its 25-year potential or starts failing in year eight.

Insulation and the warm-deck build-up

Almost every new extension roof we build is a warm deck: the insulation sits above the deck and below the membrane, with the vapour control layer underneath. That keeps the structural timber on the warm side and removes the interstitial condensation risk that plagues old cold-deck garage roofs. Expect 120–150mm of PIR to hit current U-value targets, which is around £28–£40 per square metre in materials alone. It also raises the finished roof height, so it needs designing in before the walls go up — retro-fitting insulation depth after the fact usually means a raised threshold or a compromised abutment.

Roof lanterns, skylights and where the money goes

Glazing is usually the biggest single line on an extension roof. A decent aluminium-framed lantern with a thermally broken frame and warm-edge units runs £900–£2,500 supplied, plus our labour to build and weather the upstand kerb. Flat rooflights are cheaper but the kerb detailing still needs doing properly. The failure we get called to most often is a lantern set on an undersized or badly weathered kerb, where water tracks under the frame and appears on the plasterboard reveal months later.

Parapets, abutments and the details that add cost

A simple three-sided extension with a trim on all edges is the cheapest to roof. Add a parapet wall and you add a coping detail, an internal upstand and often a hidden gutter — a genuine extra of several hundred pounds. Where the extension abuts the main house, the membrane must be dressed up the wall by at least 150mm and terminated properly, ideally into a chase or under a lead cover flashing rather than relying on a surface bead of mastic. Mastic-only terminations are the most common defect we find on extensions built five to ten years ago.

What we'd ask before signing off a builder's roofing line

Which membrane, and who is the manufacturer? What insulation thickness and resulting U-value? What fall is being designed, and how is it achieved? Who supplies and fits the glazing kerb? Is the abutment lead or membrane? Is there a manufacturer-backed guarantee, and is the installer registered to issue it? Five questions, five minutes, and they will tell you more about the quality of the job than the headline price does.

Getting a comparable quote locally

We're happy to price the roof element of an extension even when another firm is doing the build — plenty of our extension work comes to us that way, particularly around Hampton, Teddington, Whitton and Fulwell where main contractors sub the roofing out. Send drawings or even a sketch with rough dimensions and we can give a realistic range before anyone visits, then a fixed price once we've seen the site and confirmed access.

FAQs

Is EPDM or fibreglass better for an extension roof?

For most domestic extensions EPDM is the sensible default: one seamless sheet, quick to install and 25 years of service. GRP suits roofs that will be walked on or that have complex upstands and a lot of penetrations, because the finish is rigid and can be trimmed neatly around detail.

Do I need building control sign-off for the roof?

Yes — an extension roof falls under Building Regulations, covering structure, U-value and weathering. Your building control officer or approved inspector will want to see the insulation before it's covered, so don't let anyone membrane over it before that inspection.

Can you roof an extension a builder has already decked?

Often yes, but we inspect the deck and the falls first. If the firrings give less than 1:80 or the ply is already weathered and swollen, we'd rather re-deck than warranty a membrane over a compromised substrate.

How long does an extension roof take?

A straightforward 15–25m² warm-deck roof is typically two to three days on site once the deck is ready: insulation and VCL one day, membrane and trims the next, glazing kerbs and flashings to finish.

Get a fast quote with a costed breakdown

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