Two basement quotations can describe the same extra space while allowing for different work. A lower total may reflect a different specification or an unresolved assumption.
Start by checking whether both contractors priced the same information and the same finished result. Then list the differences that need a written answer.
This guide is for homeowners, developers and professional advisers comparing proposals. It helps you organise the questions; it does not provide a valuation or recommend a contractor.
2. Compare the work behind the total
Make a comparison sheet with one row for each work package and one column for each quotation. Record the document or answer supporting every entry.
Include site setup, access, excavation, spoil removal, structural work, temporary works, waterproofing, drainage, services, finishes and making good where relevant.
For each row, use a clear status: included, excluded, allowed for provisionally, or not stated. Do not treat silence as inclusion.
Check the boundaries between packages. Who forms openings, seals service penetrations, reinstates finishes and deals with work outside the new basement?
If a proposal excludes an item, record who is expected to provide it. A complete project budget may involve costs outside the main contractor's quotation.
3. Ask what could change the price
Identify provisional allowances, quantities that may be remeasured and work priced on assumptions. Ask what evidence would turn each allowance into a defined scope.
Questions may concern ground conditions, groundwater, existing foundations, obstructions, services, spoil handling or working hours. Use the actual site information to decide which questions matter.
Ask how a change would be described, agreed and priced. Refer contractual wording to the appointed contract adviser or legal adviser where needed.
Check whether VAT, professional fees and other client costs are included or excluded. Record the stated basis without guessing the tax treatment.
4. Read the sequence alongside the price
A list of permanent materials does not explain how the basement will be built. Ask for the proposed sequence and the information it relies on.
HSE's domestic basement guidance highlights the need to plan excavation and support of existing structures. Its temporary works guidance also explains the need for appropriate design and checking.
Ask who will design and check temporary works, who coordinates them with the permanent structure, and which decisions must be resolved before work starts.
Compare access, deliveries, storage and spoil removal assumptions. Ask what the programme depends on, including information from other members of the project team.
These are questions for competent appointed professionals. A quotation comparison cannot establish that an excavation method or temporary support arrangement is safe.
5. Make responsibilities and handover visible
Ask for named responsibilities for design, coordination, installation, inspection and handover. A statement that a system is included does not explain those roles.
For waterproofing, check the proposed design basis, interfaces with the structure and any drainage or maintenance requirements. Read the separate waterproofing proposal questions below.
List the drawings, records, certificates, operating information and warranties that the contractor proposes to provide. Check who supplies them and when.
Read warranties and insurance information on their own terms. Ask the relevant adviser to clarify cover, exclusions and the party responsible for responding to a problem.
6. Resolve the differences before choosing
Send each contractor a short clarification schedule. Keep the question, answer, quotation revision and any remaining action together.
Do not add guessed figures to make the totals look comparable. Ask for a revised quotation or identify the uncertainty for your appointed cost adviser.
The useful comparison is the agreed scope, the remaining uncertainty and the responsibilities behind each price. The lowest total alone does not answer those questions.
If the proposals still conflict, an independent construction review can help identify scope gaps, assumptions and interfaces to take back to the team.
Bring the proposals and the decision
Darren reviews basement contractor proposals as an independent construction adviser. He can identify matters to clarify with the contractor and appointed professionals.
He does not replace the quantity surveyor, contract administrator, engineer or legal adviser. Any formal cost advice, contract decision or design approval stays with the appropriate appointment.
Send the quotations, the information they were based on and the decision you need to make. You do not need every document before making contact.
Sources and scope
The Basement Consultant
Sources checked 3 September 2026.
HSE: Domestic basement construction
This article provides general questions for a project discussion. The answers depend on the site, proposed work and appointed professionals.