Pick a size and spec, then tick the extras your quote leaves out. The calculator gives a low–high budget using published 2026 price guides, and shows where the money goes.
How the calculator works
The room price is the outside size (length × width) multiplied by a rate per m² for the spec you pick. That is how the price guides work. The rates are BestBuilders’ 2026 bands: basic £1,000–£1,400, standard insulated £1,500–£2,000, high spec £2,000–£2,500 and bespoke £2,500–£3,500+ per m². London and the South East add 15% to 25%, the range less.co.uk and BestBuilders both use.
Extras are added only if you tick them, so nothing is counted twice:
- Base: ground screws £30–£50 per m² and a concrete slab £80–£120 per m² (MyBuilder, BestBuilders); a plastic grid £200–£600 for light buildings only.
- Power from the house: our worked totals for a contractor-dug trench, armoured cable, consumer unit, circuits and certification: about £970–£1,430 for 10m, £1,430–£2,040 for 25m and £2,040–£2,800 for 40m. Other lengths are interpolated.
- Bifold doors: £1,100–£2,000 more than a standard door (MyJobQuote prices).
- Heating: panel heater £150–£500; air-to-air heat pump £1,500–£3,000 (BestBuilders, less.co.uk).
- Toilet with gravity drainage: £3,200–£6,000, our sum from MyBuilder, less.co.uk and Summerhouse24 figures.
- Decking: softwood £70–£110, composite £90–£180 and hardwood £130–£200 per m² installed (less.co.uk).
- Site preparation £500–£2,500 (MyJobQuote); wired internet £100–£300 (less.co.uk).
- England fees from April 2026: lawful development certificate £136, planning application £272. Fees rise each April.
- Contingency: 10% on the low figure and 15% on the high one (BestBuilders recommends 10–15%).
Totals are rounded to the nearest £100. The building regulations note estimates the inside floor area by taking 12cm off each wall. Check yours: building regulations use the internal area.
What the numbers can’t tell you
- Guides disagree by thousands of pounds for the same size, so treat the result as a budget range, not a quote.
- Most guides don’t say whether their per-m² rates include VAT, the base or electrics. Ask every installer what their price includes.
- Small rooms cost more per m², because the door, consumer unit, cable run and crew cost about the same at any size.
- Sloping or clay ground can push a base towards £4,500.
Questions
How much does a 4m x 3m garden room cost?
With standard insulation it’s about £18,000–£24,000 for the room, or roughly £21,000–£29,500 once you add power run 15m from the house and a 10–15% contingency. London and the South East add 15% to 25%.
Does the calculator include VAT?
It follows the guides, and most don’t say. Assume VAT may be on top and ask each installer. New buildings are normally standard-rated at 20%.
Why is my quote higher than the calculator?
Usually the base, the cable run, access or a high spec. Compare quotes line by line, and tick the extras above that your quote includes so you compare like with like.