Free planning tool
Sketch your proposed extension on your existing floorplan. See an instant build cost range. No sign-up, no download — just a simple browser tool to help you visualise what's possible.
Drop a JPEG or PNG of your existing floorplan, or click to browse. Most homeowners find theirs in their estate agent listing or mortgage paperwork.
Type the actual distance between the two points you clicked. This calibrates the scale for everything you draw.
Pick the type of project so we can give you the right cost rates as you sketch. You can change this any time.
A free estimate is a great start — but for a project worth £40k+ you'll want a proper fixed price for the drawings, plus a designer who's done it 1,200 times. That's where we come in.
Five quick questions, fixed-price quote in under 45 seconds. No card, no call.
20 minutes on video with our founder Dave. Talk through your sketch and your options.
Book a call →If you're thinking about extending your home, the most useful first step isn't a builder's quote or a planning application — it's a sketch on your existing floorplan. It clarifies what you actually want, reveals what's physically possible, and gives you a rough budget before you commit to anything.
This free floor planner tool lets you do exactly that. Drop in your existing floorplan (you'll find one in your estate agent listing, mortgage paperwork, or by asking your conveyancer), set the scale, and sketch your proposed addition. The tool calculates the new floor area in square metres and shows a build cost range based on real-world data from over 1,200 UK projects.
The tool's estimates are based on industry-standard build cost rates per m² of internal floor area, adjusted for region and finish level. As a rough orientation:
So a typical 25 m² rear extension at standard finish in the Midlands works out at £60,000-£72,500. Add a kitchen-fit, bifolds and a slate-roof skylight and you're closer to the premium tier — around £75,000-£95,000. These numbers exclude professional fees (drawings, structural engineer, building control), which our drawings packages cover from £1,698.
For most UK homeowners the answer is maybe not. Many single-storey extensions fall under permitted development rights, which let you build without a full planning application — provided you stay within strict rules on depth, height, materials and proximity to boundaries. Loft conversions follow a similar pattern: most rear dormers qualify under PD, but anything in a conservation area or on a listed building usually needs full permission.
Our extension planner helps you visualise the rough size and shape — but the planning question is about the specifics. We do a free feasibility check at the start of every project to confirm exactly which route applies to your home.
The figure is a complete build cost for the new extension to your chosen finish level — not a shell-only number. At Standard finish, that typically means:
Premium and Luxury tiers step up the spec — better windows (e.g. aluminium bifolds), underfloor heating, higher-quality plumbing & electrical fixtures, oak doors, etc.
The figure does not include kitchen units & appliances (typically £8k–£25k), bathroom suite & tiling (£4k–£12k), floor finishes like wood / tile / carpet (£2k–£6k), or any alterations to your existing home. It also doesn't include professional fees — drawings, structural engineer, building control sign-off — which typically add £3k–£6k on top.
For a tighter quote on your specific design — and proper itemised build cost — get an instant price from our team.