Flat-roof rear extension

Flat-roof rear extensions

Most rear extensions have a flat roof because it stays below first-floor windows, keeps the eaves within permitted development limits and takes a roof lantern neatly. A modern warm flat roof, insulated above the deck and finished in single-ply membrane, GRP or felt, lasts decades when the falls and outlets are right.

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Flat roof of a new rear extension with a grey membrane, aluminium coping and a large roof lantern

What makes a good flat roof?

Falls of at least 1 in 80 as built, insulation above the deck so the timber stays warm and dry, outlets that cannot block, and upstands at least 150 mm above the finished surface where the roof meets walls and lanterns. Those details matter more than the membrane brand.

A parapet hides the roof edge and gives a crisp line; an overhanging fascia is simpler and sheds water away from the walls. We draw both for you to compare.

How much does a flat-roof extension cost?

A flat-roof single-storey rear extension is priced at £1,900–£3,100 per square metre including VAT for a standard specification; a 3 m × 4 m extension typically comes to £23,000–£37,000. A roof lantern adds about £2,200 to £4,500.

Building Regulations for flat roofs

New fabric must meet Approved Document L (2021 edition incorporating 2023 amendments): walls 0.18, roof 0.15 and floor 0.18 W/m²K, windows 1.4 W/m²K or Band B. Under the simple route, new windows, doors and rooflights should not exceed 25% of the extension floor area plus any openings removed; more glass is possible with a whole-house calculation.

Different rules in Wales, Scotland and Northern Ireland. The limits above are for England. Check your own house with our depth checker or read how far you can extend without planning.

What is included?

Warm roof build-up with insulation above the deck
Single-ply, GRP or felt membrane
Parapet or fascia edge
Roof lantern or flat rooflights
Rainwater outlets and downpipes
Lead or membrane flashings to the house
Ventilation where needed
Building control inspection

Guide prices by region

Per square metre of new floor, including VAT, standard specification.

Region	Per m² inc VAT
London	£2,470–£4,030
South East England	£2,130–£3,470
East of England	£2,010–£3,290
South West England	£1,980–£3,220
West Midlands	£1,900–£3,100
East Midlands	£1,840–£3,010
North West England	£1,790–£2,910
Yorkshire and the Humber	£1,770–£2,880
North East England	£1,710–£2,790
Wales	£1,770–£2,880
Scotland	£1,800–£2,940
Northern Ireland	£1,650–£2,700

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FAQ

Flat-roof rear extension questions

How long does a flat roof last?

A well-built single-ply or GRP warm roof typically lasts 25 years or more.

Is a flat roof really flat?

No; it is built to falls so water drains to outlets or gutters.

Can I walk on it?

Not as a balcony without planning permission and a guarding design; it can be accessed for maintenance.

Parapet or fascia?

A parapet gives a cleaner look; a fascia is cheaper and simpler to drain.

Can I add solar panels?

Often, on frames that do not pierce the membrane. They may need planning in some areas.

Will it be warm enough?

Yes; a warm roof is insulated to the Building Regulations target.

Where