Use coordinated drawings and itemised quotations, including fees, VAT and work to the existing house. RICS and the Federation of Master Builders recommend defining the work and comparing detailed prices rather than relying on a broad estimate. [1] [2]
Compare what each option actually includes
For a rear extension, identify the new envelope and the opening into the house. For a side-return extension, show the existing wing, roof junctions and any walls being removed. For a wraparound, price the combined layout and structural arrangement, not two unrelated additions. For a two-storey extension, include the upper floor, roof, upstairs alterations and supports for the complete structure. The engineer's assessment of the existing building and the new load paths is part of establishing that scope. [3]
These are comparison questions, not a ranking from cheapest to most expensive. Give each contractor the same drawings, specification and schedule of work, and ask them to identify assumptions and exclusions. [1] [2]
Investigate costs hidden below the sketch
Ground conditions, trees, nearby structures and existing foundations can affect the foundation design. Underground drainage may also require investigation and a different structural solution. Do not price a standard foundation and treat its suitability as already established. [4] [3]
A public sewer or lateral drain can require Thames Water's agreement and coordinated drainage work. Its position and the necessary access arrangements should be established before committing to the footprint. [5]
Ask the budget to distinguish new construction from alterations to existing rooms, kitchen and bathroom fittings, services, decoration and external making-good. Identify separately anything the homeowner will purchase or arrange. A quote cannot be compared fairly where these items are included by one contractor and excluded by another. [1]
Fees, VAT and possible charges
From 1 April 2026, the standard statutory householder application fee for an extension or alteration to one dwellinghouse is £548. That is not the price of the drawings or the whole approvals process. Planning Portal applies its own service charge to relevant online submissions, and building control charges are agreed separately with the chosen body. [6] [7]
HMRC states that most building work on houses and flats is charged at the standard 20% VAT rate, with specific exceptions. Ask whether each quotation includes VAT and what basis supports any claimed relief. [8]
Residential extensions under 100 square metres, where the development does not create a new dwelling, benefit from the CIL minor-development exemption. A larger extension to a qualifying principal residence may obtain a separate residential-extension exemption, with a claim required before commencement. Check the correct route with the collecting authority rather than assuming that planning approval settles CIL. [9]
Keep the budget current
Ask when quotations expire, how changes will be agreed and what allowance remains for unresolved work. Include any temporary accommodation in the household's overall budget. Compare quotations with consistent scope rather than treating a generic London construction-price range as a project benchmark. [1][2]
Draft fee regulations propose changes from 8 December 2026; use the final approved schedule for the submission date. Current fees also require a further check for annual indexation from 1 April 2027. [10][11]