Worked example: 6m × 5m = 30m² · rear extension · UK average
Quick budgeting answer: Our UK-average model places this example at £105,500, with lower and higher construction-rate scenarios of £79,000–£132,000. 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.
Why 30m² may be more than “20m² plus half”
Some costs increase with area, such as the main floor slab, roof covering and wall construction. Others are fixed or project-specific: a structural engineer’s design, site setup or the width of a new opening. The illustrative model gives £70,500 for a 20m² rear extension and £105,500 for 30m² under the same general assumptions. It is a planning comparison, not a linear rate you should apply to every property.
| Example | Model expected budget | What changes |
|---|---|---|
| 20m² rear: 5m × 4m | £70,500 | Smaller floor area, with many fixed-cost items still present |
| 30m² rear: 6m × 5m | £105,500 | Larger area and roof, potentially greater structural span |
| 30m² wraparound: 6m × 5m | £119,000 | Additional assumed complexity factor for a combined side-and-rear layout |
Four specification choices to resolve first
Ask how the roof will be supported and where any rooflights or lanterns can sit.
Large sliders or bifolds can substantially affect structural work and the glazing budget.
Plan lighting, sockets, radiator moves or underfloor heating before the concrete and plaster go in.
A finished kitchen extension includes more items than a general-purpose room with basic finishes.
Reduce uncertainty before asking for tenders
- Measure the actual available footprint, allowing for boundaries and access.
- Ask for a basic layout that shows furniture, circulation, daylight and the connection to the original house.
- Confirm likely drainage changes and the width of any load-bearing wall removal.
- Provide every builder with the same drawings, glazing schedule and finishes specification.
A long, narrow 30m² extension and a near-square 30m² extension may perform differently for daylight and furnishing. Get the room arrangement right before treating floor area as the sole decision.
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 is an example of 30m² dimensions?
A rectangular 6m × 5m footprint is 30m², as is 10m × 3m. The proportions matter for structure, garden depth and daylight even when the area matches.
Is the £105,500 example a builder quote?
No. It is the expected scenario from an illustrative model with stated VAT, fee and contingency assumptions, not an offer from a contractor.
Does the worked budget include fitted kitchen units?
No. It assumes the standard extension shell and internal finish. Select the kitchen fit-out option for a kitchen-project illustration.
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.