A useful London basement budget needs a defined site, structural solution and finished specification. A cellar fit-out, a floor-lowering scheme and a new excavation are different projects. Define which of these you are pricing before comparing quotations. RICS recommends resolving the scope and comparing quotations on a consistent basis. [1]
What drives the construction price?
The starting condition matters. Work beneath an existing building needs a method that maintains its stability, while an excavation must also deal with neighbouring structures, ground conditions, services and water. Those requirements affect the work to be priced before the internal finishes are considered. [2] [3]
Access and site management belong in the same budget. Spoil removal, deliveries and the working arrangements for a constrained street need to comply with the relevant local construction controls. Westminster and Kensington & Chelsea publish specific code and management-plan requirements for relevant projects. [4] [5]
Separate the budget into clear packages
Ask quotations to distinguish the investigation and professional design, the structural shell, waterproofing and drainage, and the finished accommodation. Within each package, record what is included, what is assumed and what remains to be priced. [1] [6]
For waterproofing, include the specified system, its interfaces and the equipment required to keep it functioning. For a pumped system, servicing and access are continuing costs rather than optional extras after completion. [7] [8]
Do not omit the technical work needed for planning. Camden's basement process, for example, includes site-specific assessment and an independent BIA audit. The required scope depends on the scheme, so request a project-specific professional quotation rather than assuming a standard report fee. [9]
VAT and approval charges
Most work to existing homes is subject to VAT at 20%. Reduced or zero rates apply only where the relevant conditions are met; calling the project a basement conversion does not itself establish an entitlement to a different rate. Make the VAT treatment explicit when comparing prices. [10]
Planning and building control costs should be identified separately from the contractor's price. Thames Water's requirements may also be relevant where the work affects a public sewer. Confirm the applicable services and charges for the actual proposal rather than inserting a generic allowance described as a fixed fee. [11] [12]
Deal with uncertainty openly
Ask the engineer and contractor which unknowns could change the scope, particularly foundations, ground and water conditions. Record how further investigation will resolve them and how any remaining provisional work will be priced. A quotation is more useful when its assumptions are visible. [2] [6]
Set the contingency against those identified risks and review it as the investigation resolves them. Keep it separate from items already included in the contractor's scope, so the same work is not counted twice.
Planning fees may change
The government's planning-fee response describes reforms subject to parliamentary approval. The parliamentary record identifies 8 December 2026 as the proposed commencement date. Obtain the made instrument and operative fee schedule before treating a replacement amount as the current charge. [13][14]
For a later application, recheck the fee schedule before finalising the approvals budget. That possible change affects the overall project allowance, not the structural feasibility of the basement. [13]