Getting and Comparing Contractor Quotes for a London Extension: The Complete Guide

The most effective way to get comparable quotes for a London extension is to prepare a clear specification document before inviting contractors to quote — and to require all contractors to price the same specification. A specification for a London extension should include: a description of all works in sequence (demolition and strip-out, groundworks and foundations, structural masonry, structural steelwork, flat roof or pitched roof construction and waterproofing, windows and doors, first-fix electrical, first-fix plumbing, insulation, plastering, second-fix carpentry, floor finishes, decoration); a list of key material assumptions (e.g., 'flat roof to be EPDM single-ply membrane on 150mm PIR insulation warm roof', 'structural steel RSJ to structural engineer's specification', 'aluminium bifold doors to rear opening by specified or equivalent supplier'); a list of client-supplied items (the items the homeowner is supplying directly — typically second-fix materials such as kitchen, sanitary ware, tiles, floor finishes — so these are excluded from the contractor's quote); and a list of explicit exclusions (items specifically not included in the contractor's scope — e.g., 'external landscaping is excluded', 'structural engineer's fee is excluded', 'Building Control fee is excluded'). When comparing quotes against a common specification: if all contractors price the same specification with the same exclusions and the same client-supplied items, the remaining difference between quotes reflects: the contractor's overhead and profit margin; the contractor's efficiency (labour productivity); the contractor's trade rates and supplier relationships; and the contractor's preliminary costs (site accommodation, waste disposal, tool hire). A specification also reduces the risk of a low-ball tender that wins on price but then recovers its margin through variations (additional-work claims for items that should have been included in a properly specified tender).

The conventional wisdom is to get three quotes — and this is a reasonable starting point, but it requires the right three contractors. The quality of the quote is more important than the number of quotes. Three well-prepared quotes from appropriate contractors are far more useful than five rushed quotes from contractors who have not visited the site. Recommended approach: identify 5-6 contractors who are appropriate for the project (relevant experience in London extensions of similar size and type, FMB or TrustMark membership, Checkatrade or RIBA-referral track record, insurance confirmation, VAT registration); send the specification document and architectural drawings to all 6 with a clear deadline for return (allow at least 2-3 weeks for a well-prepared quote on a medium extension); expect 3-4 responses that are well-prepared (some will decline or not respond); use the 3-4 responses that are fully priced and include a breakdown for comparison. The purpose of multiple quotes: verifying that the market cost range for your specification is approximately where the design and budget assumptions suggested; identifying an outlier (a significantly low quote is as concerning as a significantly high quote — both deserve investigation); providing a commercial basis for negotiation with the preferred contractor; and cross-referencing the quality of different contractors' tender packages (a detailed, well-broken-down quote from a contractor who has understood the specification is a more reliable cost indicator than a one-line quote with no breakdown).

The following are significant red flags in a contractor's quote or tender process that should prompt further investigation before contract award: A quote with no breakdown or very limited breakdown: a contractor who provides a single-line 'Supply and install rear extension: £85,000' with no breakdown of labour, materials, preliminaries, or provisional sums has not demonstrated that they have actually priced the project. Ask for a breakdown by work stage or trade. If the contractor cannot or will not provide a breakdown, this is a concern — it may indicate they intend to manage the project informally, or that their figure is a rough estimate rather than a considered price. A quote significantly lower than others (more than 15-20% below the next lowest): the most common explanation for a very low quote is that the contractor has missed or excluded something significant from their scope — materials assumed client-supplied that other contractors included, a structural element that they have not priced, or an element of the preliminaries (scaffold, waste, power and water supply) that they have omitted. Ask the low-quoting contractor to confirm specifically that their quote includes structural steelwork to the engineer's specification, all flat roof waterproofing, Building Control application, and waste disposal — and compare their confirmation against the other quotes. A quote that is missing explicit exclusions: a quote that does not clearly state what is excluded (structural engineer's fee, kitchen, tiles, floor finishes, landscaping) may include or exclude different items than the other quotes. Ask each contractor to provide a list of specific exclusions. Excessive variation claims during works: some contractors with a history of low-bid, high-variation project management provide artificially low initial quotes and then claim for additional works throughout the project — recovering their margin through variations that the homeowner has not budgeted for. Check contractor references specifically for this pattern. Request to pay for materials in advance (large advance payments beyond deposit): a contractor who asks for payment for materials that have not arrived on site, or for a stage payment before the milestone is visually complete, may have cash flow issues. Pay on milestone completion, not on request.

Once you have three or more quotes against the same specification, the tender appraisal is the process of comparing them on a like-for-like basis and arriving at a shortlist for further discussion or negotiation. Tender appraisal structure for a London extension: Step 1 — check scope completeness: for each quote, confirm that all the major work stages in the specification are addressed. Create a simple table with work stages as rows and contractors as columns. Mark each cell as 'included', 'excluded', or 'provisional' for each contractor. Identify any stage where one contractor has excluded or marked as provisional something that other contractors have included as fixed — this is a scope difference that invalidates direct price comparison until it is resolved. Step 2 — normalise provisional sums: provisional sums (allow items for structural steelwork, for structural engineer, for Building Control, for unknown groundworks conditions) should ideally be the same amount across all quotes if the specification provides a prescribed provisional amount. Where contractors have set their own provisional sums at different levels, normalise them to the same amount for comparison purposes — then re-rank by adjusted total. Step 3 — compare preliminaries: ask each contractor to confirm whether their quote includes scaffold (if required), waste disposal for the full project, temporary power and water to site, and protection and reinstatement of existing landscaping. These items can each add £2,000-£6,000 to the true project cost if not included in the tender. Step 4 — assess commercial terms: compare payment structure (deposit and milestone payments), whether a retention is proposed, the defects liability period offered, and whether the contractor uses a formal contract (JCT or standard bespoke). A contractor proposing a JCT Homeowner Contract and a 5% retention for 6 months is demonstrating commercial discipline. Step 5 — assess the contractor's quality: the final decision between closely-priced contractors should include: quality of the quote document itself; willingness to provide references and to arrange reference property visits; contractor's current programme (when can they start?); and your direct assessment from site visit and communication of the contractor's professionalism and communication style.