How Much Does an Extension Cost in London?

Test a straightforward 30 m² extension at an illustrative £2,500–£4,000/m² excluding VAT: that gives £75,000–£120,000 for the assumed construction scope. If the works are standard-rated at 20%, the equivalent construction allowance including that VAT is £90,000–£144,000. Establish the actual tax treatment with the contractor or adviser.

Before using that arithmetic, specify whether the price includes demolition, structural alterations to the existing house, kitchen units, glazing, drainage, decoration and landscaping. Complex ground conditions, basement work, restricted access and premium finishes can move the cost well outside that example.

Allow separately for the measured survey, architectural concept and planning drawings, detailed building regulations design and construction information. Ask which revisions, coordination meetings, tender support and site services are included.

Structural design and calculations are another budget line, with investigation, specialist foundations or temporary-works design identified separately where necessary. Use a written, project-specific proposal rather than assuming a single percentage covers every discipline.

For a £100,000 construction input, the current Shorplans fee calculator gives an indicative combined architectural/structural fee of £4,500 before VAT, using its 4.5% band. Its displayed stage allocation is £1,800 planning/survey design, £1,575 building-regulations technical design and £1,125 structural package. These are the calculator's indicative allocations, not a binding appointment or separate statutory charges.

That building-regulations design allocation is not the building control body's application and inspection charge. Add the actual council or approver quotation separately. Likewise, add independent party wall surveyor quotations, council application fees, specialist investigations and any construction-stage services outside the design appointment. Confirm inclusions and VAT in a written proposal; do not double-count a structural package already included.

Planning or lawful development application fees are paid to the relevant authority and change over time. Building control charges depend on the provider, scope and application route. Tree surveys, drainage investigations, sewer agreements and specialist reports may also be needed.

Where the Party Wall Act applies, budget for independent surveyor fees and potential adjoining-owner appointments, not just a standard notice letter. The number of affected owners and the complexity of the work make a large difference. Shorplans can coordinate with surveyors but does not provide party wall surveying.

For an early feasibility budget, test a contingency of 10–15% of the relevant construction allowance, with more where investigations are incomplete or the existing building is uncertain. This is a budgeting assumption, not a cap on unexpected cost.

Add temporary accommodation, storage, insurance changes and finance costs where relevant. Map deposits and stage payments against the contract and avoid paying large amounts ahead of demonstrable progress.

Issue the same drawings, specification and return schedule to each contractor. Ask them to identify exclusions, provisional sums, VAT, programme, payment terms and responsibility for temporary works.

Clarify unusually low allowances before appointing. A well-described scope gives a more useful comparison than three headline totals built on different assumptions. Reconcile the chosen tender with your contingency before starting.