Three builders can look at the same extension and produce three very different totals. The easy conclusion is that one is expensive, one is cheap and one sits somewhere in the middle.

Often the more important question is whether they have priced the same job.

The Federation of Master Builders recommends getting detailed written quotes from at least three builders and giving each of them the same brief, drawings, specification and site information. Citizens Advice gives similar guidance and stresses the distinction between a quote — an agreed price for defined work — and an estimate, which is a best guess.

Before comparing the totals, put the quotes side by side and compare these 10 lines.

Start with the most basic question: what is each builder actually agreeing to do?

A quote for an extension might say “build rear extension to drawings”, but the drawings rarely tell the entire commercial story. Does the price include demolition? Making good to the existing house? Structural openings? Plastering? Decorating? Kitchen installation? External drainage? Reinstating paving or landscaping?

Two quotes can both look complete while one stops at a shell and the other includes far more finishing work.

Use the same written brief for every builder, then mark each quote as Included, Excluded, Allowance or Not stated against the main work packages.

Do not move on until you understand the gaps. A total is not comparable while the scope is different.

A quote should make clear which labour and materials are included.

This matters most where homeowners plan to buy some items themselves. You may be supplying the kitchen, sanitaryware, floor finish or light fittings, but who supplies the adhesive, trims, fixings, waste fittings, sealants or connection materials that go with them?

Responsibility can become blurred at the edges.

If you are supplying a product, confirm who is responsible for checking dimensions, receiving it, storing it safely and dealing with damage or missing parts. A builder may quite reasonably exclude the time and risk created by owner-supplied products, but that needs to be visible before the price is accepted.

A lower quote can simply contain a longer shopping list for you.

Check whether VAT is included in the headline total and, where it applies, at what rate.

The Federation of Master Builders and Citizens Advice both recommend making VAT explicit in the written quote. That sounds obvious, but a price written as “£60,000 + VAT” is not comparable with “£68,000 including VAT”.

Do not assume VAT treatment from the size of the number or from what another builder has done.

For ordinary home-improvement work, the tax position can depend on the type of work and circumstances. If a reduced or zero rate is being claimed, ask the contractor to explain the basis rather than building your budget around an assumption.

The comparison figure in your spreadsheet should be the amount you realistically expect to pay, including VAT where due.

A quote can include “bathroom fittings”, “flooring” or “windows” without telling you what quality has actually been allowed for.

Ask for named products where practical, or at least a clear allowance.

If Builder A has allowed £25/m² for tiles and Builder B has allowed £70/m², the difference in their totals may have little to do with labour or efficiency. They have priced different specifications.

The same issue appears with doors, ironmongery, taps, sockets, roof windows, radiators, sanitaryware, insulation products and kitchen worktops.

Where a final choice has not been made, write down the allowance and whether it covers supply only or supply and installation.

Then compare allowances separately from the fixed construction work. That makes it much easier to see where your own later choices can move the budget.

A provisional sum is money set aside for work or a product that cannot yet be priced accurately.

It can be legitimate. The danger is forgetting that it is uncertain.

Groundworks are a common example because the full condition of the ground may not be known until excavation. Existing drainage can create the same problem. Specialist joinery, utility diversions or restoration work can also be carried as allowances while details are unresolved.

If one builder has £15,000 of provisional sums and another has £3,000, their bottom-line totals do not carry the same level of certainty.

Create a separate “uncertain” total for each quote. Ask what evidence or design work is needed to convert those allowances into fixed prices and when that can happen.

The quote with the lowest total but the largest unresolved allowance can easily become the most expensive later.

Read the exclusions before the inclusions.

The FMB's current guidance specifically tells homeowners to check waste removal, and larger projects can also involve skips, scaffolding, plant, temporary protection, welfare facilities, parking or access arrangements.

Some builders roll these costs into their price. Others show them separately. Some expect the homeowner to arrange particular items.

None of those approaches is automatically wrong.

The risk is that a cost exists but no one has taken responsibility for it.

Ask who pays for skips and disposal; who provides scaffolding; whether protection to existing floors and rooms is included; whether parking permits or restricted-access costs have been allowed for; and who reinstates damage around the working area.

A short exclusion can carry a large future bill.

A main contractor may employ electricians, plumbers, roofers, heating engineers or other specialists as subcontractors. Alternatively, some of those trades may sit outside the quote.

Make that clear.

Who arranges the electrician? Is testing and certification included? Who coordinates building-control inspections? If there is an air-source heat pump, solar installation or specialist glazing package, is that within the builder's responsibility or directly contracted by you?

This is not only a price issue. It affects programme, communication and responsibility when one piece of work depends on another.

If a builder excludes a specialist package, add the separate quotation to your comparison so the total remains meaningful.

Also confirm which completion documents you expect to receive. The exact certificates depend on the work, but “finished” should include the evidence needed for the installations that were actually carried out.

Price is only one part of a quote. The FMB recommends checking the anticipated start date and duration too.

A builder who can begin next week is not automatically a better choice than one who is booked for three months. A very quick start can be perfectly legitimate, but it is worth understanding the reason.

Ask for the expected start window, the estimated duration and any important assumptions behind the programme.

Will the job run continuously? Are there known lead times for windows, steel, cabinetry or other long-lead items? Is the builder relying on you to make product selections by particular dates?

A realistic programme should also acknowledge that weather, hidden conditions and approved variations can affect building work.

The useful comparison is not simply “12 weeks versus 16”. It is whether the programme is credible and whether you understand what could change it.

Do not compare only the final total; compare when you are expected to pay it.

Citizens Advice recommends getting payment arrangements into the written contract, while Northern Ireland's nidirect guidance cautions against paying large deposits or the whole price upfront.

For a substantial renovation, payments are commonly staged as work progresses. The important point is that the stages are understandable and related to the project rather than being a vague sequence of dates.

Ask what deposit is required, when each payment becomes due and what work should have been completed by that point.

Also ask how the final payment and any snagging or outstanding work are handled.

Cash flow matters to the builder, but it matters to you too. A quote that requires much more money early in the job exposes you differently from one with a more balanced schedule, even if both totals are identical.

The final line is the one that protects the project once reality starts changing the drawings.

Citizens Advice says a written quote is an agreed price for the specified work, but extra work can legitimately change the amount where you request it or where additional work is identified and you agree to pay for it. GOV.UK consumer-contract guidance also warns against unfair terms that effectively give a trader an unrestricted right to change the agreed price.

Before appointing a builder, ask how variations will work.

A sensible process might require a written description of the change, its cost and any programme effect before the extra work proceeds, except where urgent safety action is genuinely necessary.

Also check how long the original quote remains valid. Material and subcontractor prices do not stay open indefinitely, and the validity period matters if you are still waiting for planning, finance or design decisions.

If a quote is due to expire before you can appoint the builder, ask for it to be refreshed. Do not assume the old price has simply been held.

The simplest way to compare three quotes is to stop treating each PDF as a self-contained document.

Create one table with the 10 lines above down the left and Builder A, Builder B and Builder C across the top.

For each cell, write one of four things:

Included.
Excluded.
Allowance.
Not stated.

Then add the actual figure or short note where relevant.

Once the table is complete, you can start comparing totals.

You may find the lowest quote is still the strongest. You may find the highest one includes thousands of pounds of work that the others have left out. You may find all three have assumed something different about the kitchen, scaffolding or electrical package.

That is the information you need before making a decision.

The Federation of Master Builders explicitly advises homeowners not to choose on price alone. A low number can represent genuine value, but it can also mean something has been omitted, underestimated or interpreted differently.

Price also cannot tell you whether the builder communicates well, has relevant experience, carries appropriate insurance, can show comparable work or has a credible team and programme.

Those checks belong beside the quote comparison, not inside it.

A detailed quotation is not a guarantee that a building project will never change. Renovations uncover things. Homeowners change their minds. Products become unavailable. Weather and access can interfere.

Its job is to make the starting agreement clear enough that when something does change, everyone knows what the original position was.

That is why the best comparison begins one level below the total.

Three different prices are not useful until you can see whether they describe the same job.