First Floor Extension Cost UK 2026: Prices + Calculator

A first floor extension costs £45,000–£98,400 including VAT in the UK in 2026, for a standard 20–25m² addition over an existing garage or ground-floor room. The same build runs £50,000–£82,000 excluding VAT (SECalcs, 2026). Per square metre, first floor extensions price at £1,650–£3,000+ — above ground-floor rates because the structure must carry itself, the existing roof must come off, and the house stays weatherproof under a temporary roof throughout.

What does a first floor extension cost per m²?

A first floor extension costs £1,650–£3,000+ per square metre, and the rate splits by what sits underneath. Building over an existing load-bearing room is the cheaper route; building over a garage adds reinforcement cost because most garage walls and foundations were never designed to carry a storey.

Build type	Cost per m² ex VAT (2026)	Why the rate differs
Over existing ground-floor room	£1,650 – £2,400	Load-bearing structure already in place; roof comes off and goes back on
Over garage (reinforcement needed)	£2,500 – £3,280	Single-skin garage walls and shallow footings need steel or new foundations
Over garage (foundations already adequate)	£1,900 – £2,600	Post-1990s garages sometimes built for future storey; engineer confirms first
London and South East premium	+20 – 40%	Labour, scaffold over street, restricted access, party wall both sides

First Floor Extension Cost Calculator

How much does it cost to build over a garage?

Building over a garage costs £50,000–£82,000 ex VAT for a 20–25m² room in 2026. The deciding line is the garage structure: a single-skin brick garage with shallow footings (most pre-1990s builds) needs steel reinforcement (£2,000–£6,000) or fresh foundations under new blockwork walls, while a garage already built to carry a future storey skips that line. A structural engineer's survey (£600–£1,200) settles which garage you have before you spend anything else. Budget the full sequence: survey, steel design, scaffold with temporary roof, structure, weatherproofing, then fit-out — the garage's old roof, door and electrics all come out as part of the works.

Do you need planning permission for a first floor extension?

Most first floor extensions need full planning permission. Permitted development covers a two-storey rear extension only up to 3m deep, with the roof matching the house, upper-floor windows obscure-glazed and side windows set at least 1.7m above floor level. Building over a garage changes the roofline and usually breaks permitted development; two-storey side extensions are excluded entirely. Where permission is needed, the householder application fee is £258 in England (2026) and the decision takes 8–13 weeks. Building regulations approval (£500–£1,000) applies to every first floor extension with no exceptions — a new storey changes the fire escape strategy for the whole house, which triggers Part B work (fire doors, interlinked alarms) even on the floors you did not touch.

What extra costs come with a first floor extension?

First floor extensions carry the standard fee set plus three lines ground-floor builds never see: scaffolding with a temporary roof, structural steel, and the re-roofing of the opening.

Extra cost	Typical fee (2026)
Architect drawings	£2,000 – £5,000
Structural engineer (survey, steel design, calculations)	£600 – £1,500
Planning application (most first floor builds)	£258 householder (England)
Building control	£500 – £1,000
Party wall surveyor (per neighbour, if applicable)	£700 – £1,500
Scaffolding with temporary roof	£3,000 – £8,000
Structural steel frame	£2,000 – £6,000
Opening and re-roofing the existing structure	£2,000 – £5,000
VAT (20%) — usually excluded from builder quotes	+20% on labour and materials

How long does a first floor extension take?

A first floor extension takes 16–24 weeks on site — longer than a ground-floor build of the same size because the roof comes off, the house lives under a temporary cover, and the structure goes up before the fit-out starts. Design, structural drawings and planning add 3–4 months up front. Full project: 6–9 months from first sketch to finished rooms.

Stage	Typical duration
Design + structural drawings	4–8 weeks
Planning permission (most first floor builds)	8–13 weeks
Builder procurement	2–4 weeks
First floor build on site	16–24 weeks

Is 50k enough for a first floor extension?

£50,000 covers a 20m² first floor extension over an existing ground-floor room at standard spec outside London (£33,000–£48,000 ex VAT) with fees and VAT inside the budget. £50,000 does not cover a build over a garage — that route runs £50,000–£82,000 ex VAT before fees. In London, add 20–40% to every line, making £50,000 a deposit rather than a budget. For cheaper ways to add an upstairs room, compare the loft conversion cost guides.

First floor extension or loft conversion — which is cheaper?

A loft conversion is cheaper: a dormer loft conversion costs £30,000–£65,000 against £45,000–£98,400 including VAT for a first floor extension, for a comparable upstairs room. The first floor extension wins in two cases: the loft is unusable (low pitch, no headroom, no space for a compliant staircase), or a single-storey wing or garage makes the structural work straightforward. The two projects share most of their planning constraints — building regulations Part B fire escape rules apply to both, and a party wall agreement is needed on terraced and semi-detached houses either way.

What adds more value, a first floor extension or a loft conversion?

Both add roughly 20% to property value when they create a double bedroom with an en-suite (Nationwide research). A first floor extension costs more to build but delivers rooms at first-floor level with conventional windows and full ceiling heights, which valuation surveys treat as fully equivalent to the original house — some loft conversions with compromised headroom value slightly below. For the full comparison, see the house extension cost guide.

Frequently asked questions