How Much Does a 3m x 5m Extension Cost in 2026?

Last updated: June 2026 · Figures checked against live North London quotes and current government fee schedules.

How much does a 3m x 5m extension cost?

Quick answer: A 3m x 5m (15m²) single-storey rear extension costs £42,000–£63,000 to build in North London in 2026, based on rates of £2,800–£4,200 per m². With design fees, approvals, party wall costs and VAT added, a realistic all-in budget is £55,000–£80,000.

Type "extension cost" into a search engine and 3m x 5m is the size Britain asks about more than any other — no accident, since 3m of depth and 5m of width maps neatly onto the rear of a typical terrace or semi. Fifteen square metres is also a sweet spot of consequence: enough to transform a cramped galley kitchen, small enough to stay inside permitted development on most houses. Here is what those fifteen metres cost in North London right now, with nothing left off the bill.

What does a 3m x 5m extension cost to build in North London?

Single-storey extension work across Barnet, Enfield, Haringey and the surrounding boroughs currently tenders at £2,800–£4,200 per m², which puts the 15m² shell-to-finished build at £42,000–£63,000. In premium pockets — Hampstead, Highgate, Muswell Hill and their neighbours — rates of £3,500–£5,000+ per m² are normal, lifting the same footprint to £52,500–£75,000 or beyond. Be aware that small extensions carry the highest rate per metre: scaffold, skips, welfare and design effort cost much the same at 15m² as at 25m², so the fixed overhead is spread across fewer metres. Anyone promising a 15m² build for £25,000 is pricing a different decade.

How does finish level change the price?

Finish level	Rate per m²	Build cost (15m²)	Typical glazing
Standard	£2,800–£3,200	£42,000–£48,000	uPVC French doors, standard rooflight
Mid-range	£3,200–£3,700	£48,000–£55,500	Aluminium bifolds, one large flat rooflight
High-spec	£3,700–£4,200	£55,500–£63,000	Slimline sliders, roof lantern, UFH throughout
Premium-area projects	£3,500–£5,000+	£52,500–£75,000+	Bespoke structural glazing common

Glazing is the lever that moves a 15m² project between bands faster than anything else: swapping £1,800 French doors for a £9,500 slider plus a £4,000 lantern adds nearly £12,000 on its own — close to £800 per m² of footprint.

What does a 3m x 5m extension cost all-in?

Build cost is never the cheque total. Here is a complete worked example for a mid-range 15m² rear extension on a North London semi with one attached neighbour: build at £3,500 per m² comes to £52,500; VAT at 20% adds £10,500, taking construction to £63,000. Design and drawings add £1,800 (the usual span is £1,200–£2,500), structural engineering £900 (range £500–£1,500), building control £1,100 (range £500–£1,500), and a party wall surveyor £1,200 (range £700–£2,500 per affected neighbour). A lawful development certificate to evidence the permitted development position costs £274. Total: £68,274. Trim the spec and you can land near £55,000; pick a premium street, a lantern and two notifiable neighbours and £80,000 arrives quickly. That £55,000–£80,000 envelope is the honest answer.

How do you keep a 3m x 5m project near the bottom of the range?

Four levers move the total more than any haggling does. Hold the rear opening to around 3m and fit French doors with sidelights rather than a full-width slider, saving £6,000–£8,000. Leave the existing drainage run where it lies, because diverting a manhole adds £1,500–£3,000. Choose a fixed flat rooflight over a lantern and bank £2,000–£3,500. Finish the floor in quality LVT at £40–£55 per m² rather than stone or oversized porcelain. None of these choices weakens the structure or shrinks the room; together they routinely pull a tender down by £10,000–£15,000 — the whole distance between the middle and the bottom of that £55,000–£80,000 envelope.

Do you need planning permission for a 3m x 5m extension?

Usually not. A 3m-deep single-storey rear extension sits within permitted development on most terraces and semis (detached houses get 4m), provided you respect the 4m eaves rule, the 3m height limit within 2m of a boundary, and materials matching the host house — and provided no Article 4 direction, conservation designation or flat/maisonette status removes your rights. The smart move is a lawful development certificate at £274, giving you council-stamped proof for future buyers. If your design strays beyond the limits, a householder planning application costs £548 — a figure worth double-checking anywhere you read it, because it climbed from £258 to £528 in April 2025 and again to £548 on 1 April 2026, leaving most cost guides two rises out of date. Apply through the Planning Portal and a £91.02 processing charge is bolted on; paying the council directly avoids it. Building regulations approval is required on every route.

What do you actually get in 15 square metres?

More than the number suggests. Across a 5m-wide rear, 3m of depth turns a 2.5m-deep galley kitchen into a 5.5m-deep kitchen-diner that seats six to eight comfortably. Alternatively, 15m² hosts a full kitchen plus a 1.8m x 2m utility room and a WC — the combination that transforms how a family house functions. The 3m depth has a quiet advantage too: daylight still reaches the middle of the original floorplan, where 5m-deep additions often leave the old rooms dim without extra rooflights. For scale, 15m² is roughly the footprint of a single garage and a half.

Frequently asked questions

Is 3m deep enough for a kitchen-diner?

Yes, when combined with the existing footprint. A 600mm worktop run, a 1.2m island, circulation and a dining zone fit comfortably once the new 3m joins the old 2.5–3m of kitchen depth — the layout most North London semis end up choosing.

How long does a 3m x 5m extension take to build?

Ten to fourteen weeks on site is realistic, plus 6–10 weeks beforehand for design, engineering and approvals. Programmes slip when groundworks surprise or glazing arrives late, so confirm lead times — sliders can take 6–8 weeks from order.

Will a 3m x 5m extension add value?

Generally, though street ceilings cap it. With much of North London trading around £7,000–£10,000 per square metre, 15m² of well-built space often appraises at or above its cost — but check local sold prices before assuming profit rather than lifestyle return.

Do I need a party wall agreement for a 3m x 5m extension?

If you excavate within 3m of a neighbour's structure and deeper than their foundations — true of most North London semis — the Party Wall Act applies. Budget £700–£2,500 per affected neighbour unless they consent in writing to your notice.

Can I build 3m x 5m under permitted development on a flat?

No. Flats and maisonettes hold no householder permitted development rights, so any extension needs a full planning application at £548. The 3m rule applies to houses only.

Want your 3m x 5m priced as one fixed, VAT-inclusive figure? Run it through the Extension Builder, then book a free site visit — we design and build across North London from Cockfosters, on 020 3051 9430.