Two storey timber frame extension
Adding a second floor to an extension is where timber frame earns its keep. Both storeys and the roof can be standing inside two weeks, where blockwork would need the scaffold lifted three or four times.
Because one roof and one set of foundations serve two floors, the rate per m² of total floor sits in the same range as a single storey, and often towards its lower half. A 20 m² footprint giving 40 m² of floor typically costs £56,000 to £96,000 including VAT.
When a second storey is worth it
- You need a bedroom or bathroom as well as ground-floor space, and moving house would cost more than building.
- The garden is small. Going up doubles the floor area on the same footprint.
- Neighbours are close. A fortnight of frame erection is easier on them than months of bricklaying at height.
- The house is detached or on a wide plot, so the planning limits for upper floors can be met.
- You want the upstairs usable quickly. Internal trades can start on both floors once the shell is wrapped.
Two storey prices
| What | Typical price including VAT |
|---|---|
| Rate per m² of total floorBoth floors added together, plain to high finish | £1,400 to £2,400 per m² |
| 30 m² over two floorsA 15 m² footprint, about 5 x 3 m | £42,000 to £72,000 |
| 40 m² over two floorsA 20 m² footprint, about 5 x 4 m | £56,000 to £96,000 |
| 60 m² over two floorsA 30 m² footprint across a detached house | £84,000 to £144,000 |
| Structural engineer, two storeysBeams, point loads and the opening into the house | £800 to £1,800 |
| Building control, two storeysPlan approval and inspections, up to about 40 m² | £900 to £1,300 |
| Party wall surveyor, per neighbour, one agreed surveyorOnly where a neighbour does not consent in writing; more if each side appoints its own | £700 to £1,500 |
About these figures
These are typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. Two-storey extensions of every construction type typically cost £1,700 to £2,900 per m² of total floor. Bathrooms, kitchens and fees are extra. Read them as a guide, not a quote: only drawings and a look at the ground give a firm enough to price your job.
Two floors, one frame. Send the footprint and we will find a firm to price it.
Get a quotePlanning limits for upper floors
Permitted development in England is tight once you go above one storey. The extension may project 3 m at most past the back wall the house was first built with, has to keep 7 m clear of the boundary at the bottom of the garden, and cannot go at the side. Where it comes within 2 m of any boundary, eaves are capped at 3 m, which in practice rules out two storeys near a fence.
The roof pitch has to match the house as far as practicable, any side window upstairs needs obscured glass and has to be fixed shut below 1.7 m, and the materials must look like the existing walls. None of it applies in conservation areas and similar designated land, where two-storey extensions always need an application.
Most two-storey schemes therefore go in as a householder planning application, with a fee of £548 in England. Councils look hardest at overlooking and at loss of light to the neighbour. A designer who knows the local policy on the 45-degree line from next door's windows will save you a refusal. Scotland's householder classes are stricter still on upper floors near boundaries, so assume an application there.
How a two-storey frame goes together
The method is called platform frame. Ground-floor wall panels are stood and braced, a floor deck of engineered joists is laid across their heads, and that deck becomes the working platform for the first-floor panels. Trusses or attic rafters finish it off.
Panels for an upper floor are too heavy and too high to hand-ball safely, so the erectors bring a telehandler or a small mobile crane for a day or two. That needs somewhere to stand: a drive, a front garden, or occasionally the road with a permit from the council. Tell the firm early if the only way round the back is a 900 mm passage.
Scaffolding goes up before the frame and stays for the cladding and roofing. It is designed to stand clear of the panels, not tied through them.
Allowing for the frame to settle
Timber shrinks a little across the grain as it dries to the humidity of a heated house, and the horizontal members in a platform frame (plates, rails and floor joists) all contribute. Over two storeys the frame can come down by several millimetres in its first couple of years. Masonry does the opposite and grows slightly.
The detailing absorbs this. Where a brick skin passes a window, a gap is left under the sill and filled with a soft sealant. The same is done at the eaves and at any point where something fixed to the frame passes through the brickwork. Engineered joists, which barely shrink, cut the allowance. NHBC publishes the gap sizes and building control inspectors look for them.
At the junction with the house, a vertical movement joint runs the full two storeys, and floor levels are set so the new first floor lines through with the old one after settlement, not before.
Fire, sound and structure
Walls and the intermediate floor of a two-storey house extension need 30 minutes of fire resistance, which a timber frame gets from its plasterboard linings. Cavity barriers close the gap behind the cladding at each floor level, around every opening and at the eaves, so a fire cannot travel unseen. They are cheap, and easy to forget on site, so ask to see them before the cladding covers them.
If the new bedroom makes the upper floor of the house larger, building control may ask for linked smoke alarms throughout and an escape window. Between rooms, the floor needs sound insulation: mineral wool between the joists and a dense plasterboard ceiling meets the standard.
Wide openings on the ground floor, such as sliding doors under a bedroom wall, put a concentrated load on the posts either side. The frame maker's engineer sizes a steel or laminated timber beam for it and thickens the foundation locally.
Neighbours and the Party Wall Act
In England and Wales, digging foundations within 3 m of a neighbouring building and deeper than its footings triggers the Party Wall etc. Act 1996. So does building up to or astride the boundary. You serve written notice at least one month before excavation starts. If the neighbour consents in writing there is nothing more to pay; if not, a surveyor draws up an award at your cost.
Timber frame does not change these duties, though its lighter loads sometimes let the engineer keep footings shallower than next door's and outside the Act. Scotland has no equivalent statute; common law and your title deeds govern.
Sources
- Householder permitted development, technical guidance (GOV.UK)
- Extensions: planning permission (Planning Portal)
- Scottish Government circular on householder permitted development
- Approved Document B on fire safety (GOV.UK)
- Party Wall etc. Act 1996 booklet (GOV.UK)
- NHBC Standards, chapter 6.2 on timber framed walls
Order of work for two storeys
Permissions
Planning consent, building regulations approval and any party wall notices. Allow three to four months before a spade goes in.
Foundations and scaffold
Footings sized for two floors, slab, sole plate kerb, then the scaffold. Panels are made during these weeks.
Frame in two lifts
Ground-floor panels, floor deck, first-floor panels and roof, usually inside two weeks with a telehandler on site.
Envelope and fit-out
Membrane, windows, roof covering and cladding outside; services, insulation and plasterboard inside on both floors at once.
Questions we get asked
How long does a two storey timber frame extension take?
About twelve to sixteen weeks on site for 40 m² of floor. In brick and block, expect a month or two longer, mostly because each lift of blockwork has to cure before the next.
Can I add a timber frame storey on top of an existing brick extension?
Often, yes, and its low weight is the reason people ask. A structural engineer first has to prove the existing foundations and walls can take the load, usually by digging a trial hole beside the footing.
Is a two storey timber frame extension cheaper than brick?
Per m² the two are close, and timber frame is often a little cheaper on paper. The bigger saving is in weeks of labour, scaffold hire and site overheads. Try both in the cost calculator.
Will a mortgage lender accept it?
Lenders treat modern timber frame with a brick or rendered face as standard construction. Keep the completion certificate and the frame maker's structural calculations with the deeds.
Do I need a crane?
Usually a telehandler, sometimes a small mobile crane for a day. If neither can reach, panels are made smaller and lifted by hand from the scaffold, which adds labour.
More to read
Get a quote for a two storey extension
Three short parts: a postcode, four questions about the job, your contact details. A minute of your time, and free.
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