A house extension quote breakdown shows what you pay for each part of the build: design and fees, groundworks and foundations, structure, roof, windows and doors, electrics and plumbing, finishes and site costs. Two quotes can differ because they cover different work, not because one builder charges more. Compare the scope first, then the price.
What Does a House Extension Quote Usually Include?
- Design and fees: drawings, structural calculations and planning applications where required.
- Site preparation: clearance, excavation and soil removal.
- Foundations and drainage: excavation, concrete, reinforcement and drainage changes.
- Structure and materials: brickwork, blockwork, steel beams, external walls and insulation.
- Roofing: roof structure, coverings, flashings, gutters, fascias and soffits.
- Windows and doors: frames, glazing, bi-fold or sliding doors and fitting.
- Electrical and plumbing: wiring, sockets, lighting, pipework and heating extensions.
- Finishing: plastering, flooring, skirting and decoration.
- Site costs: labour, scaffolding, skip hire and waste disposal.
Tick each item off for every quote you receive. A gap is a question to ask the builder, not proof of a problem. Gaps spotted early can save disputes later.
What Is Often Excluded From an Extension Quote?
Check whether these are listed or left out: architect and engineer fees, planning and building control charges, Party Wall surveyor costs where they apply, kitchen or bathroom fittings, landscaping and external repairs, and final decoration. Rules and fees differ across the UK, so check GOV.UK or your local authority.
Which Hidden Costs Can Raise Your Extension Budget?
- Unexpected ground conditions.
- Existing drains that clash with the plans.
- Extra structural support.
- Specification upgrades and design changes.
- Extra work found once building starts.
Ask each builder how they price changes, and consider keeping a contingency for surprises.
How to Compare Two House Extension Quotes
- Make sure both quotes cover the same design and specification.
- Check labour, materials and quantities.
- Review provisional sums, which are placeholders for work not yet priced.
- List every exclusion.
- Compare VAT treatment, the total payable and payment stages.
Example: Comparing Two Extension Quotes
This is a fictional example. It uses no real prices.
| Cost category | Builder A | Builder B | What to check |
|---|---|---|---|
| Foundations | Included | Provisional sum | Depth and concrete specification |
| Structural steel | Included | Not specified | Beam specification and fitting |
| Roofing | Standard finish | Premium finish | Material and insulation |
| Windows | Standard units | Upgrade extra | Frame type and glazing |
| Plumbing | Excluded | Included | Connections and fixture allowances |
| Scaffolding | Included | Excluded | Hire duration |
| Waste removal | Not specified | Included | Skip hire and disposal |
| VAT | Excluded | Included | VAT basis and total payable |
Builder A includes scaffolding but leaves out plumbing and VAT. Builder B includes plumbing and VAT but treats foundations as a provisional sum. Neither is clearly cheaper until you add the missing items and confirm the unclear ones in writing.
Why Do Extension Quotes Differ So Much?
Extensions are made of many parts, and each builder may price, include or exclude them differently. One quote may assume a simple build, while another may allow for premium finishes or tougher ground. Different allowances and exclusions can move the total noticeably, even when the design looks the same on paper.
Questions to Ask Your Builder
- Which items are included, excluded or provisional?
- Who handles building control and other approvals?
- What happens if the ground or structure needs extra work?
- How are changes priced and agreed in writing?
How ScopeCheck Helps You Compare Extension Quotes
ScopeCheck is an AI-assisted tool for UK homeowners. You add two to five quotes for the same job as PDFs, photos or pasted text. It shows what each quote includes, what is missing and what to ask the trader. VAT is added the same way to every quote, and each line points to the quote's own wording. See how it works.
You can start with a free preview, with no sign-up. A full report is £29 for one project, or £69 for three comparisons. These are launch prices and rise to £39 and £79 on 1 January 2027. See the pricing page. ScopeCheck does not take money from traders, and your quotes are not shared with them. It is a decision aid, not a survey. AI can miss details, so always check the original wording.
Frequently Asked Questions
What should be included in a house extension quote?
Design and fees, groundworks, foundations, structure, roofing, windows and doors, electrics, plumbing, finishes and site costs. Exclusions and VAT should be stated clearly.
Are foundations included in a house extension quote?
Usually, but check. Some quotes treat foundations as a provisional sum because ground conditions are uncertain. Ask what depth and specification are assumed.
Are architect fees included in building quotes?
Often not. Design and professional fees are frequently separate, so ask who is responsible for drawings, calculations and approvals.
Does a house extension quote include VAT?
It should say so clearly. VAT treatment can differ, so ask each builder to confirm the VAT basis and the total payable.
Why do two builders give different extension quotes?
They may price different scopes, specifications, allowances and exclusions. Compare like for like before judging the price.
Final Thoughts
A good quote breakdown makes the scope clear before the price. Check every major item, ask about every gap and get answers in writing. Then you can compare extension quotes with confidence.
Want to compare your quotes more clearly? Try the free preview and see what each quote really includes.