Describe your extension and receive a clearer initial cost guide.
Tell us what you're planning in your own words. We'll identify the important details, ask a few relevant questions and use the Builder's Brief UK calculator to prepare an initial cost breakdown. It's a budgeting range — not a builder's quote.
At a glance
- What this helps with
- Turning a plain-English description of your extension into a structured cost range you can use for early budgeting.
- Best for
- UK homeowners at the idea stage who aren't sure how to describe their project or which options affect the cost.
- What you get
- • Your description turned into structured project details
- • A few clarifying questions where details are missing
- • A cost range from the Builder's Brief UK calculator
- • A category breakdown you can download as a PDF
- 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.
Include the extension type, rough size, where you are in the UK, finish level, and anything special (kitchen, bifolds, wall removals, tricky access). Skip anything you don't know — we'll ask.
Terms we'll use as we go
Builder-grade materials — laminate flooring, painted walls, standard sockets. Suitable for rental or budget refresh.
Mid-range materials from mainstream UK suppliers. Engineered wood, decent tiling, standard kitchen units.
Higher-end materials throughout — quality solid wood or stone floors, bespoke joinery, high-end kitchen and lighting.
Builder can bring materials and skips through a side gate or driveway without carrying through the house.
Materials must go through the house or up narrow paths. Adds 5–10% to labour costs.
A load-bearing wall. Removing one needs a structural engineer, steel beam and building control sign-off.
Architect, structural engineer, building control, party wall surveyor. Usually 10–15% on top of the build cost.
A safety buffer for unexpected costs — usually 10–20% of the total. Older homes and unusual sites need more.
Most VAT-registered UK builders charge 20% VAT on labour and materials. Unregistered builders don't, but check they're legitimate.
You don't yet know if the ground is stable, sloping, or has old drains/foundations underneath. Push contingency higher until surveyed.