Compare extension quotes, line by line

“Build extension” does not define where a builder’s contract ends. One offer may cover a weatherproof shell. Another includes plastering, heating and decorating. A third assumes you will appoint an electrician and buy the doors. The totals mean little until those boundaries are clear.

Use one coordinated drawing pack and a written finish schedule. Record revisions and structural information used by each builder. Where design work is incomplete, ask the builder to state assumptions rather than silently filling the gaps. Early estimates and fully specified quotations carry different levels of uncertainty.

Describe handover in ordinary words: shell, plastered or ready to occupy. Include existing rooms affected by new openings and service routes. Compare what you will actually receive, not the page count of the quote, and obtain independent professional support where the project is complex.

What a complete extension quote should include

• Document revisions — List drawings, engineering information and specifications used for the price. Identify anything still awaiting design or confirmation.
• Completion standard — Define the end point: shell, plastered or finished. Explain kitchen, bathroom and decorating limits in an agreed written schedule.
• Site arrangements — Include access, welfare, storage, security and protection while your home remains occupied, with responsibilities for shared routes.
• Demolition — Describe removals, temporary support and disposal. Clarify protection and making good where existing walls or ceilings are opened.
• Foundations — State design and excavation assumptions. Establish how unexpected ground conditions are evidenced, assessed and priced before work continues.
• Drainage — Identify investigations, diversions, connections and inspection access, along with any permissions or decisions not yet resolved.
• Structure — Include steels, load-bearing alterations, temporary works and reinstatement. Allocate engineering and site responsibilities rather than assuming they overlap.
• Walls and roof — Specify construction, insulation, coverings, weatherproofing and external finish, including where the extension meets the existing building.
• Windows and doors — Identify products, dimensions, glazing and fitting. Say who completes final measurements and orders replacements if anything is unsuitable.
• Services — Describe electrical, water and heating changes, controls and testing, including alterations outside the new room.
• Internal finishes — List plaster, floors, skirting, paint and fitted rooms. Keep homeowner purchases and provisional selections separate from confirmed work.
• Approvals and handover — Allocate inspections, fees, certification, snagging, waste and outside reinstatement. Check the requirements relevant to your UK nation and property.

What's often left out — and what it can cost

The single-storey reference below is a broad per-area extension guide. It does not prove a builder includes all finishes or your chosen kitchen. Ask what area has been used and what completion standard the quote describes.

There are no dedicated extension benchmarks here for unforeseen excavation, a drainage diversion, an individual steel, design fees or decorating. Keep those uncertainties visible and get specific prices rather than manufacturing component figures from the overall range.

A shell quote and a ready-to-use room are different offers. Do not add unrelated whole-kitchen or window packages to an extension range without checking overlap. This guide is a prompt for a project-specific conversation, not a bill of quantities or professional cost plan.

An illustrative extension comparison

An absent reinstatement line may leave you arranging another trade; uncertain foundations may mean additional work within the build. Treat those risks separately. Ask for clear boundaries and a written approval process rather than a blanket promise that everything is included.

Questions to ask each trader

• Which drawing revisions and structural details have you priced, and what remains unresolved?
• Exactly what will be finished at handover, including affected rooms in the existing house?
• Which figures are provisional, and how will ground conditions or selections alter the price?
• What VAT treatment have you used, and is the final total inclusive?
• What deposit is proposed and which observable completed stages trigger later payments?
• Who manages approvals, inspections and written agreement of cost and programme changes?

Your extension questions

Resolve the contract boundary before ranking totals. Give every builder the same drawing pack, finish schedule and change procedure so you understand both the promised outcome and the remaining uncertainties.