Project cost research
Work out a realistic cost range before you request quotes
Headline £/m² figures are a starting point, not a budget. Use the AI cost estimator to build a range from your own project details, then see the extra costs that sit outside the builder's price.
Describe your project and get an indicative range · Spreadsheet and printable planner — £19.99
At a glance
- What this helps with
- Getting a sensible cost range for an extension or renovation before you approach builders, and understanding what moves that range.
- Best for
- UK homeowners at the research stage who want a defensible starting number rather than a rumour from a forum.
- What you get
- • An indicative range from the AI cost estimator
- • Why single £/m² figures mislead
- • The inputs that move cost most
- • Costs that sit outside the builder's price
- • How to plan contingency sensibly
- Important disclaimer
- The Builder's Brief UK provides practical homeowner guidance only. It is not legal, financial, architectural, structural engineering, planning or building-control advice. Always use suitably qualified professionals where required.
Why a single £/m² figure is only a starting point
The same floor area can cost very differently depending on decisions that have nothing to do with size.
- Ground conditions and foundation depth are only known once you dig
- Structural openings into the existing house vary hugely in cost
- Glazing choice can move a project by thousands on its own
- Roof type, and whether it is flat, pitched or has rooflights
- Access: a rear garden with no side gate changes labour and muck-away
- Finish level, from builder-standard to bespoke
- Regional labour rates and current trade availability
Costs that sit outside the builder's price
These are real project costs. Leaving them out is the most common reason a budget fails midway.
- Design and drawings, and structural engineer calculations
- Planning application or lawful development certificate fees where relevant
- Building control application and inspections
- Party wall matters where they apply
- Kitchens, bathrooms, sanitaryware, flooring and decoration
- Furniture, blinds, landscaping and making good the garden
- VAT, where it applies to your project
- Contingency for the unknowns you cannot price in advance
How to use an estimate properly
- Treat the output as a range, not a quote — real prices come from builders who have seen the site
- Re-run the estimator as decisions firm up; ranges narrow as the specification does
- Record your assumptions so you can explain them to builders
- Use the range to sense-check quotes rather than to negotiate a price down
A sensible research sequence
1. Describe the project honestly
Rough size, storeys, access, glazing ambitions and finish level. Vague inputs produce vague ranges.
2. Read the range, not the midpoint
The low end usually assumes simple structure and modest finishes; the high end assumes the opposite.
3. Add the costs outside the build
Fees, building control, kitchens and finishes belong in the project budget, not in a footnote.
4. Add contingency before you commit
A contingency you have already set aside turns a surprise into an inconvenience rather than a crisis.
Next step
Describe your project and get an indicative range
Launch the AI cost estimatorCommon questions
How much does a single-storey extension cost in the UK?
Is the estimate a quote?
Why do the estimator's figures differ from a builder's quote?
How much contingency should I plan for?
Does the estimate include VAT and professional fees?
Also useful
The Builder's Brief UK provides practical homeowner guidance only. It is not legal, financial, architectural, structural engineering, planning or building-control advice. Always use suitably qualified professionals where required.