What is a fixed-price building contract vs a quote?

One of the most expensive misunderstandings in building is treating a quote as if it were a binding price. They are not the same thing, and the gap between them can cost real money. Knowing the difference between a quote and a fixed price building contract protects you from a price that quietly balloons after you have committed.

A quote is an estimate, not a commitment

A quote is essentially a sales figure, an estimate of what your build might cost based on a set of assumptions. It has no legal force on its own. It can change once the site is assessed, the inclusions are firmed up, or the design is finalised. A quote is useful for comparing builders at a glance, but it is not what you are actually committing to.

A fixed price contract is the binding agreement

A fixed price, or lump sum, building contract is the real thing: a binding legal agreement that locks in the price, the inclusions, the schedule of progress payments and a construction timeframe, signed before construction begins. This is what governs your build and what you are legally committed to, which is why it deserves far more attention than the quote that preceded it.

Why the difference matters for your budget

The risk is committing on the strength of an attractive quote, only to find the contract price is higher once everything is firmed up, or that the build costs balloon because the quote left things out. The contract is what you pay, not the quote. So the number that matters for your budget is the fixed price in the contract, not the figure on the marketing material.

Why lenders need the contract, not the quote

Your lender feels the same way. A lender funds a build against the fixed price building contract, not a quote, because it needs a known total to calculate how much to lend and how to schedule the progress payments. An estimate is not something a lender can lend against. This is one of the practical reasons you cannot finance a build on a quote alone, as our guide on how a construction loan works explains.

Watch the inclusions and exclusions

A low quote often looks attractive precisely because it excludes things, site costs, finishes, or generous allowances that will later be exceeded. When you move from quotes to contracts, compare what is actually included on a like for like basis. The cheapest quote frequently becomes the dearest build once the exclusions catch up with you.

Fixed price is not fully fixed

It is important to know that even a fixed price contract is not completely frozen. Variations you request after signing, and provisional sums and prime cost allowances that come in higher than estimated, can still move the final figure. Fixed price limits the increases to defined circumstances, it does not eliminate them, which is covered in detail in our fixed price building contract guide.

Cost plus is different again

There is a third arrangement to be aware of: a cost plus contract, where you pay the builder actual costs as they are incurred plus a margin. With cost plus the final price is unknown, which is why most mainstream lenders will not fund a build on that basis. For a standard first home build, a fixed price contract is what you and your lender will expect.

How to use this when comparing builders

The practical takeaway is not to choose a builder on quotes alone. Get to the contract and inclusions stage with your shortlist, compare what each fixed price actually covers, and scrutinise the provisional sums. That is the only way to compare builders fairly, because two quotes that look similar can hide very different contracts.

Protect yourself before you sign

Before you sign a building contract, have it reviewed, ideally by someone who knows building contracts, understand how variations are handled, and never rely on a verbal quote or a handshake. The contract is the document that governs your build, your budget and your finance, so treating it with the seriousness it deserves is one of the most important things you can do.