Start with the requirements and affordability of your particular project; RICS advises considering these alongside the property and the benefits you need. [1]
Compare the work, not just its label
For an extension, ask the engineer what is known about foundations, ground conditions and the opening into the existing house. For a loft, ask about the proposed floor, roof structure and stair arrangement. These are separate engineering questions, so do not assume that two similarly sized schemes require the same structural work. [2]
For a basement option, request a separate feasibility and cost assessment rather than adapting an extension allowance. RICS highlights the complexity of basement projects and the importance of specialist advice. [1]
To make the comparison useful, specify the intended rooms, finishes and fitted items. Ask each estimator to identify what their price assumes about access, temporary protection, excavation, structural work, drainage, services and making good. Leave an unanswered item as an identified allowance or exclusion, not an assumed free extra.
Build a whole-project budget
Use separate budget headings for:
- Construction, including the agreed fixtures and finishes.
- Design, engineering, surveys and other specialist services.
- Council and other approval charges, plus any infrastructure liability.
- VAT, client purchases, accommodation and an allowance for identified uncertainty.
Professional fees may be fixed, percentage-based or time-charged, and should be distinguished from payments made to public authorities. Ask which basis applies and what changes could alter the fee. [3]
Most building work to existing houses and flats is standard-rated at 20% VAT. Do not compare a VAT-exclusive quotation with an inclusive budget or assume an extension receives the new-home treatment. [4]
Make early estimates explicit
Ask for an estimate date, drawing revision and list of assumptions. Request a written distinction between a preliminary budget and the price offered for a defined construction contract. RICS procurement guidance identifies the importance of the selected procurement route and the information available when obtaining prices. [5]
For your own comparison, place quotations side by side and add excluded items back in using separately identified estimates. A lower headline total is not the lower complete total when the scope differs.
Check the less visible cost questions
Where Thames Water is the sewerage undertaker, building over or near relevant public drainage can require its agreement or a different solution. Establish the sewer position before treating the proposed footprint as settled. [6]
Ask whether any remaining uncertainty could change the design rather than merely the price. Agree when that uncertainty will be investigated and who must authorise extra expenditure.
Use a budget tied to your actual drawings, specification and exclusions. Keep the construction total separate from the complete amount you need to fund, and identify the decisions that could change either figure before committing to the work.