Estimate extension and refurbishment costs at concept stage
Proper Job creates an early UK building-cost range from the project details you know, with assumptions, exclusions and unresolved decisions kept visible. Move to a drawing-based Detailed Estimate when sufficient information is available.
What information do I need?
Start with the type of work, approximate area, postcode area, finish and known site conditions. Include work to the existing house, not just the new extension. Structural details, drainage and final specifications may still need confirmation.
If you have concept drawings, use the details they establish for a Guide Price. A Detailed Estimate is a separate step: upload PDF drawings, review the supported measurements and use them to produce itemised work packages. Unmeasured or undecided work remains an allowance or item to confirm.
See the output
Kitchen extension: early range, assumptions and exclusions · Refurbishment: drawing-based Detailed Estimate
Both are demos, not evidence of achieved building costs.
What changes the range?
Location, structure, access, drainage, glazing, kitchen and bathroom allowances, services and work to the existing house. Check the stated scope, VAT, exclusions and unresolved decisions before comparing prices.
How the Guide Price is calculated
Proper Job uses stored rates and calculation rules: base rates or work-package costs, regional and finish adjustments, specific allowances, site setup, risk and margin. Missing drawings and known constraints affect the risk allowance and range. The calculation records which version of the rates and rules was used, alongside the inputs and allowances.
These are not live builder quotations. The range is for planning, not a guarantee that the final cost will fall within it.
Why use this rather than a generic AI answer or simple calculator?
An area-only calculator cannot distinguish projects with the same floor area but different access, structural work or kitchen allowances. Proper Job includes these details in a structured calculation and keeps the scope and unknowns visible.
Compared with asking ChatGPT for a ballpark figure without a supplied costing method, the advantage is a recorded pricing basis you can inspect and revise. ChatGPT and other tools can also use detailed inputs and cost data. More cost drivers alone do not prove greater accuracy; that requires comparison with like-for-like quotes or completed-project costs.
Price
First Guide Price free, then £9.99 inc VAT. Detailed Estimate £29.99 inc VAT.
When do I need a builder or QS?
A Guide Price is an early planning range, not a fixed quote or an independently verified cost plan. A builder needs to check the property and scope before quoting. For substantial or complex work, an independent residential quantity surveyor can review the budget before you commit to the design.