Modular extension cost by size
Floor area is the first thing a builder asks and the biggest single driver of the price. These are typical UK figures for nine sizes, with a note on what each size is good for.
The table
Typical national ranges for 2026, VAT included. Expect 20 to 40% more in London and the South East. A guide, not a quote.
| Floor area | The room | With fees | With fees and a contingency |
|---|---|---|---|
| 10 m² | £21,000 to £33,000 | £23,500 to £38,000 | £26,000 to £43,500 |
| 12 m² | £25,000 to £39,500 | £27,500 to £44,500 | £30,500 to £51,000 |
| 15 m² | £31,500 to £49,500 | £34,000 to £54,500 | £37,500 to £62,500 |
| 20 m² | £42,000 to £66,000 | £44,500 to £71,000 | £49,000 to £81,500 |
| 25 m² | £52,500 to £82,500 | £55,000 to £87,500 | £60,500 to £100,500 |
| 30 m² | £63,000 to £99,000 | £65,500 to £104,000 | £72,000 to £119,500 |
| 35 m² | £73,500 to £115,500 | £76,000 to £120,500 | £83,500 to £138,500 |
| 40 m² | £84,000 to £132,000 | £86,500 to £137,000 | £95,000 to £157,500 |
| 50 m² | £105,000 to £165,000 | £107,500 to £170,000 | £118,500 to £195,500 |
All columns include VAT. The middle column adds £2,300 to £4,800 for drawings, a structural engineer and building control; the last keeps 10 to 15% on top for surprises. For London, a kitchen or a lift over the house, use the calculator.
10 to 15 m²
A room of 3 m by 4 m takes a utility and a downstairs WC, a desk and a sofa bed, or a dining table for six. It travels as a single module on an ordinary lorry and is usually lifted in within a morning.
At 3 m deep it fits permitted development on almost any house in England, so there is often no planning application to pay for. The price per metre is at its highest here, for the reasons on the cost per m² page.
20 to 25 m²
This is the size most people have in mind: a kitchen pushed out by four metres, with a table by the doors. It is also where transport starts to shape the design. A load wider than 2.9 m counts as abnormal and its haulier must notify the police and highway authorities before moving it, so makers either keep each module under that width or build the room as two pieces.
Four metres deep is within permitted development for a detached house. On a semi or a terrace it needs prior approval, a lighter application that costs £249 in England and gives the neighbours a chance to comment.
30 to 35 m²
Kitchen, island, table and sofa in one room, typically 5 m by 6 m. Expect two modules, a longer day for the crane and a joint in the floor, walls and roof to be finished on site. None of that is a problem, but it adds days at your house and can mean a second day of crane hire at about £1,400.
A room this deep goes beyond the basic permitted development limits on most houses, so allow for either prior approval or a householder application at £548.
40 m² and over
Now the extension is a wing: a kitchen and living space with a utility behind, or an annexe-sized room with its own shower. Budgets pass £100,000 once fees are added, and the single-storey rate is no longer the only way to price it. Panels assembled on site, or a hybrid with a masonry base, start to compete with whole modules, and a designer earns their fee in working out which.
Permitted development also limits an extension to half the land around the original house, a rule that small gardens meet sooner than their owners expect.
Choosing the size on price
Each extra square metre adds £2,100 to £3,300. Going from 20 m² to 25 m² therefore adds £10,500 to £16,500, which is about the cost of a mid-range fitted kitchen. Before paying for more floor, lay the furniture out on paper at the smaller size. A well-planned 20 m² often lives better than a loose 25.
The cheapest metre is the one that keeps you inside a threshold: under the permitted development depth, under the width that travels as one module, clear of a drain or a tree. Half a metre less depth can remove a planning application and a second lorry at a stroke.
Scotland: the permitted development limits above are England's. Scottish rules are set out separately, and every extension there needs a building warrant from the council before work begins.
Where these figures come from
Prices are the national 2026 range of £2,100 to £3,300 per m², VAT included, from UK cost guides, multiplied by floor area and rounded to the nearest £500. Fees are typical published ranges; building control fees are set by each council. Planning limits, fees and the abnormal load width are from the pages below.
Size and price questions
How much does a 20 m² modular extension cost?
£42,000 to £66,000 in 2026 with VAT included, for the finished room craned into place. Add £2,300 to £4,800 for drawings, an engineer and building control.
How much does a 30 m² modular extension cost?
£63,000 to £99,000 with VAT included. At this size the room often travels as two modules.
What is the smallest modular extension worth building?
Around 8 to 10 m², enough for a utility room or a small office. Below that the crane, the lorry and the opening into the house make up so much of the bill that a site-built room is usually cheaper.
How big can I go without planning permission?
In England a single-storey rear extension can reach 3 m from the back wall of a semi-detached or terraced house, or 4 m from a detached one, as permitted development. Up to 6 m and 8 m is possible with prior approval from the council. Scotland and Wales set their own limits.
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