Worked example: 5m × 4m = 20m² · rear extension · UK average
Quick budgeting answer: Our UK-average model places this example at £70,500, with lower and higher construction-rate scenarios of £52,500–£88,000. These are planning allowances, not a narrow predicted contractor quote.
Illustrative model results for the precise example shown, not current quotes from builders. Budgets include an assumed 20% VAT on construction, professional/approval allowances and 10% contingency. A survey or different specifications could change them.
How we calculated the 20m² scenario
The selected inputs are a single-storey rear extension, 5 metres by 4 metres, UK-average region, contemporary/standard finish and normal access. The example contains no optional rooflights, large sliding doors, new kitchen or structural-opening surcharge. The model adds professional fees, a standard VAT allowance on construction and contingency. See all model assumptions.
| Possible footprint | New floor area | Design implication |
|---|---|---|
| 5m wide × 4m deep | 20m² | A relatively broad rear opening may need careful structural design |
| 4m wide × 5m deep | 20m² | Deeper into the garden; daylight and boundary positions need thought |
| 10m wide × 2m deep | 20m² | Long and shallow; extensive glazing and roof geometry could change costs |
Why the same 20m² does not mean the same quote
A narrow doorway is different from removing most of the wall for an open-plan kitchen.
Garden access, spoil removal and excavation constraints are hard to price from area alone.
Bespoke glazing packages can have a disproportionate effect on a modest extension.
Painting, flooring, heating changes and a new kitchen may be outside the basic quote.
Compare 15m² and 20m² before settling on a plan
For a 5m × 3m rear addition, the same model gives an expected planning budget of £52,500, compared with £70,500 for 20m². That difference is illustrative, not a promise that every builder will price the extra metre of depth the same way. Think about whether the extra space changes what the room can actually do.
Ask for the building price, professional costs, VAT, contingency and any kitchen/flooring upgrades as separate lines. That makes competing proposals easier to assess.
Calculate your own extension budget
The examples above describe specific assumptions. Change the dimensions, region, finish and extras to explore your project.
Calculate my extension price →Common questions
What dimensions give 20 square metres?
Examples include 5m × 4m, 10m × 2m or 6.67m × 3m, although practical layouts, wall thickness and planning considerations vary.
Does £70,500 include a new kitchen?
No. The expected example is for a general rear extension using standard internal finishes, without the optional new-kitchen fitting allowance.
Is a 20m² extension always cheaper than 30m²?
Usually the smaller building uses fewer materials, but some expenses such as professional work, site setup and the structural opening do not scale proportionally.
Pricing context: Federation of Master Builders construction benchmark; the full-budget numbers are generated by our published illustrative model, which makes additional assumptions. For permissions in England see Planning Portal. Other UK nations have different rules. Always obtain site-specific professional advice and comparable written builder quotes.