Choosing the cheapest builder’s quote can be tempting, but the lowest total is not always the lowest cost. Two builders may appear to be pricing the same extension while making different assumptions about the design, finishes, site conditions or work included.
RICS advises homeowners to have major extension work fully designed so builders can provide comparable quotes, and recommends obtaining at least three detailed quotes. It also advises using a written contract and a fully itemised, costed schedule of works. You can read the RICS guide to home extensions and improvements.
Start with the same information
Fair comparison begins before the quotes arrive. Issue each builder with the same information and ask them to confirm exactly what they have priced.
- The latest architectural drawings, including the revision number.
- Structural engineer’s drawings and calculations, where applicable.
- A written specification covering construction standards and finishes.
- Information about planning permission, building regulations and any party wall arrangements.
- Relevant surveys, such as drainage, asbestos, ground or building condition surveys.
- Your required start date, completion target and any restrictions on working hours.
RICS tendering guidance states that competing contractors should be given identical tender documentation when a like-for-like price is required. It also recommends keeping a record of tender queries and answers, because important clarifications may need to form part of the final contract documents. See the RICS guidance on tendering strategies.
Use a comparison checklist
1. Scope of work
Check whether each quote covers the whole project or only the main building work. Look for separate allowances or omissions relating to:
- Site setup, protection and temporary works.
- Demolition, strip-out and removal of waste.
- Foundations, drainage and connections to existing services.
- Structural steelwork, making good and alterations to the existing house.
- Windows, doors, roofing, insulation and ventilation.
- Plastering, flooring, decorating and final cleaning.
- Kitchen, bathroom, heating and electrical installations.
- External paving, landscaping, fencing and reinstatement.
A quote that says “build extension as drawings” may still leave important details open to interpretation. Ask the builder to provide a written schedule of works or mark the drawings and specification to show what is included.
2. Drawings and specifications
Confirm that every builder has priced the same drawing revisions and specifications. An earlier drawing may show fewer structural details, a different window arrangement or lower-cost finishes.
Ask each builder to identify any design responsibility they have accepted. For example, will they design drainage runs, temporary works, kitchen layouts or mechanical and electrical installations, or are those items to be completed by others?
3. Labour and materials
Check whether labour is included for every listed trade and whether materials are identified by quality or product standard. “Sanitaryware included” is less useful than a description of the expected range, quantity and installation requirements.
Ask whether the price includes delivery, storage, plant, access equipment, protection of existing areas and disposal of packaging. Also check whether the builder expects you to buy any fixtures, fittings or finishes directly. If so, confirm who is responsible for delivery, checking quantities and installing defective or late items.
4. Preliminaries
Preliminaries are the costs of running and managing the site rather than forming part of the permanent building. They can include site supervision, welfare facilities, scaffolding, hoarding, temporary services, security, protection, insurance-related requirements, surveys, cleaning and waste disposal.
Do not compare one quote’s detailed preliminaries with another quote that simply includes a single unexplained allowance. RICS guidance recommends that preliminaries are presented in a consistent format so they can be analysed and ring-fenced. Ask what is included, how long the allowance covers and what happens if the programme is extended.
5. Provisional sums
A provisional sum is an allowance for work or materials that cannot yet be priced with certainty. Common examples may include uncertain ground conditions, drainage diversions, specialist joinery or finishes that have not been selected.
Every provisional sum should be listed, described and identified as defined or undefined where relevant. Ask:
- What exact work does the allowance cover?
- Does it include labour, materials, plant, overheads and profit?
- What information was used to calculate it?
- What happens if the actual cost is higher or lower?
- Can the item be designed, surveyed or selected before contract so the allowance can be reduced?
RICS says contractors should review whether provisional sums are adequate based on the information available. If one builder has used much lower allowances, their quote may look cheaper while carrying more cost risk for you.
6. VAT
Check whether every figure is stated inclusive or exclusive of VAT. RICS reminds homeowners that VAT will generally apply to an extension or improvement unless the work falls within a specific exception, such as a new-build house. Do not compare a VAT-exclusive total with a VAT-inclusive total.
7. Programme and completion
Ask for the proposed start date, construction period and expected completion date. Clarify whether the programme assumes planning, building regulations, structural information, client selections or utility connections are already in place.
Ask what events could extend the programme and how the builder will notify you. The contract should record agreed start and completion dates, together with a method for dealing with changes. RICS notes that changes after work starts are usually more expensive, so decisions should be made as early as possible.
8. Payment terms
Check the deposit, stage payments, valuation process, payment periods and any retention. Payments should relate to identifiable progress rather than being made in advance without protection. Ask whether materials paid for before installation become your property and whether supporting invoices or valuations will be provided.
RICS recommends agreeing payment arrangements in writing and says homeowners should not pay cash in advance on the promise of a cheap deal. The Planning Portal also explains that a building contract can set out payment arrangements, insurance details and completion dates. Its building contracts guidance is an introductory source, not a substitute for legal advice.
9. Assumptions and exclusions
Read the exclusions section before looking at the final total. Typical exclusions may relate to:
- Planning, building control or statutory fees.
- Architectural, engineering, surveying or party wall costs.
- Upgrades selected after the quote was prepared.
- Unforeseen ground conditions, asbestos or defective existing work.
- Utility company charges and service diversions.
- Kitchen appliances, light fittings, floor coverings or decorations.
- External works, landscaping and repairs outside the defined area.
- Temporary accommodation, storage or additional security.
Ask the builder to list assumptions as well as exclusions. An assumption such as “existing drainage is suitable” can become a variation if it proves incorrect. Request a written price or agreed method for dealing with each uncertain item.
Why the cheapest quote may not be comparable
A low quote may reflect missing work, lower material allowances, fewer preliminaries, unrealistic provisional sums or a shorter programme that does not account for the actual constraints of the site. It may also contain qualifications that transfer design or construction risk back to you.
Prepare a simple comparison table with one row for each work package and columns for each builder. Record the quoted amount, allowance, inclusion, exclusion and question. Then compare the totals after adding VAT and making equivalent allowances. Do not silently adjust the quotes yourself; issue the same clarification to all builders and keep their written responses.
If you would value a second view before submitting or comparing a tender, tell us about your project and we can suggest the most useful level of support.
When a measured estimate or tender review could help
A measured estimate can be useful when the design is developed enough to quantify the work but builders’ quotes differ widely or are difficult to analyse. It can provide a structured benchmark by breaking the project into measured work sections, materials, labour and allowances. It cannot remove uncertainty where the design or site information is incomplete.
A tender review is particularly helpful when you have several detailed returns but need an independent comparison of scope, qualifications, provisional sums, preliminaries, programme and payment terms. A professional can also identify whether a clarification should be issued to every builder and whether the tender information is sufficiently complete for a safer appointment decision.
Before signing, make sure the agreed quote, drawings, specification, schedule of works, clarifications, exclusions, programme and payment terms are attached to the written contract. If important information is still unresolved, obtain suitable professional or legal advice rather than relying on the headline price alone.
