What the guide price covers
The per-square-metre rates are for a complete room: strip foundations and a ground slab, the fabricated frame, insulated walls, a flat or low-pitched roof, glazing, electrics, heating, plaster and a floor ready for its finish. The calculator then adds a structural engineer and building control, because no steel frame is built without either.
Kitchens, bathrooms and floor finishes are left out, since they vary by tens of thousands. So are the items in the table below, which depend on your plot and your neighbours.
Costs that sit on top
| Item | Typical cost including VAT |
|---|---|
| Structural engineer Design of the frame, connections and foundations, single storey | £600 to £1,200 |
| Building control Plan check and site inspections, single storey of 10 to 40 m²; each council sets its own fees | £700 to £1,100 |
| Architect or designer Planning drawings; a full service is usually 3% to 15% of the build cost | £1,000 to £2,500 |
| Party wall surveyor Per neighbour, only if they do not consent to your notice; one agreed surveyor, more if each side appoints its own | £700 to £1,500 |
| Householder planning application England, where permitted development does not cover the design | £548 |
| Raft foundation in place of strip footings Soft or made-up ground | £3,000 to £6,000 |
| Piled foundations Poor ground or large trees close by | Priced by your engineer after a ground investigation; can add several thousand pounds |
What moves the price
The width of the openings
Span drives the weight of steel. A beam over a 3 m opening is light enough for two people to lift. Over 6 m it is several times heavier, its columns need bigger pads, and the sliding doors beneath it are the most expensive item in the room. If the budget is tight, a 4 m opening with a fixed pane beside it gives much the same view.
How much steel is on show
Hidden steel gets a coat of primer and a plasterboard casing. Exposed steel needs neat welds ground smooth, a paint system applied in the workshop and intumescent coating where fire resistance is required. Expect the visible version of the same frame to cost noticeably more.
Access
Steel that can be carried down a side passage and bolted up by hand is cheap to erect. A mid-terrace with no rear access needs the frame designed in short pieces, or a crane and a road closure.
The ground
A steel frame puts its load on a few points, not along a wall, so each column wants a concrete pad sized by the engineer. On firm ground that is routine. Clay with trees nearby, or filled ground, can mean a raft or piles, and that is the commonest reason a quote comes back above the guide. Piles are designed and priced by the engineer after a ground investigation, and can add several thousand pounds.
Region
Labour rates in London and the South East usually push prices up by 20 to 40%. Elsewhere in the UK the spread is narrower than people expect.
Steel against brick and block on cost
A masonry extension runs £1,800 to £3,000 per m² of floor, so the steel-framed version of a plain room usually costs more. The comparison shifts once the design has a wide opening or a corner of glass. A masonry build then needs a goalpost or box frame of its own, and the saving shrinks to little or nothing, while the steel frame still goes up in days instead of weeks.
If your design is a simple room with ordinary doors, the honest advice is to build it in blockwork and spend the difference on better glazing.
How to read a quote
- Check whether the steel is priced from the engineer’s drawings or from an allowance. An allowance can move.
- Look for the foundations as a separate line, with the assumed depth stated.
- Ask who supplies and fits the glazing. It is often a separate contract with its own lead time.
- Confirm that fire protection to the steel, making good to the house and building control fees are in or out.
Where these figures come from
Prices are typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. A guide, not a quote. The planning fee, set by MHCLG from 1 April 2026, and the rules mentioned come from the documents below.
- Council planning fee schedules for England, 2026 to 2027
- Permitted development rights for householders: technical guidance (GOV.UK)
- Building regulations approval (GOV.UK)
- Approved Document A: structure (GOV.UK)
- The Party Wall etc. Act 1996: explanatory booklet (GOV.UK)