The short answer
On cost they are close. Published 2026 UK guides put a modular single-storey rear extension at about £2,100 to £3,300 per m² and a traditional one at about £2,200 to £3,300, because foundations, drainage and finishes cost the same either way. Modular wins clearly on time, with two to six weeks on site against ten to sixteen, while traditional gives more freedom over shape and materials.
On this page
Most people come to modular extensions expecting them to be much cheaper. The honest answer from published price guides is that the build cost is close, and the real difference is time.
Side by side
| Modular (volumetric or panel) | Traditional brick and block | |
|---|---|---|
| Single storey, installed, per m² | about £2,100 to £3,300 | about £2,200 to £3,300 |
| Time on site | 2 to 6 weeks | 10 to 16 weeks |
| Whole programme after design | 6 to 10 weeks | 12 to 18 weeks |
| Weather risk | Low once the shell is set | Wet trades slowed by winter |
| Shape and design | Best as rectangles; sizes set by transport | Almost any shape |
| Access needed | Crane, or a side gate for panels | Skips, deliveries, scaffold |
| Planning rules | Same | Same |
| Building regulations | Same | Same |
Figures are typical ranges from published UK builder price guides, 2026, before VAT.
Why the costs end up so close
Roughly a fifth of a single-storey extension’s cost is foundations and groundworks, and that is the same whichever way the walls are built. The opening into the house, the steel beam over it, the drainage, the plastering where old meets new, the kitchen, the flooring and the decoration are also largely the same.
The factory-built shell is where modular saves: one published worked example put the superstructure about £3,800 cheaper for a 25 m² extension. Then the crane, transport and delivery add back around 5% of the total. The net difference in that example was about 2% in favour of modular.
Where modular clearly wins
Time. Two to six weeks of builders at the house instead of three or four months. If you are living in the house, that is the difference between a disrupted month and a disrupted season.
Predictability. Most of the work happens indoors on a production line, so rain does not stop it and the price is usually fixed earlier.
Quality control. Walls are built square, insulation is fitted in the dry and airtightness can be tested before the module leaves the factory. Meeting the fabric standards in Approved Document L is straightforward.
Less mess. Fewer deliveries, less spoil, no mixers and fewer weeks of skip and scaffold hire.
Where traditional still wins
Shape. Irregular plots, wraparounds, angled walls and joining to awkward existing rooflines are easier to build on site.
Matching materials. If the council expects full brick to match the house, a traditional outer leaf does it naturally. Modular systems use brick slips or a separate brick skin.
Access. A terrace with no side access and a street where a crane cannot stand may rule out whole modules, though panel systems avoid that.
Later changes. Cutting a new opening into blockwork is routine. In a factory panel it needs the engineer.
Planning and building control are the same
Neither the planning system nor building control treats modular differently. The permitted development limits on depth, height and materials apply to both, and both need building regulations approval and a completion certificate.
So which should you choose?
Choose modular when time and disruption matter most, the design is a clean rectangle and there is access for a crane or panel delivery. Choose traditional when the shape is complex, the site is very tight or matching brick is essential. Either way, get quotes on the same specification so you are comparing like with like.
Run both scenarios in the cost calculator, or see the methods compared on our modular extension page. When you want prices, the form below finds a specialist who covers your area.