What a fixed price does and does not cover
A fixed price is fixed against a specific set of drawings and a specific list of assumptions. Understanding allowances, provisional sums and variations is the difference between a surprise and a decision.
"Fixed price" is the most misread phrase in residential construction. It does not mean the final cost cannot change. It means the builder has committed to a price for a defined scope, and that anything outside that scope is a separate conversation.
That is not a loophole. It is the only honest way to price work where some of the information does not exist yet.
Allowances
An allowance is money set aside for something you have not chosen. Tapware, tiles, lighting, appliances: the quote carries a figure so the total is realistic, and the figure is replaced by the real cost once you select.
The question to ask is not "how much is the allowance" but "what does this allowance buy". An allowance that matches the fittings in the drawings is useful. One set low to make a quote look competitive guarantees a shortfall later.
Provisional sums
A provisional sum covers work whose extent nobody can see yet. Ground conditions, drainage, rectifying what is found behind a wall. It is priced when the work is exposed and understood.
Good practice is to name what the sum assumes: rock-free excavation to a stated depth, or a slab in sound condition. Then when reality differs, you can see exactly what changed rather than being handed a bigger number.
Variations
A variation is a change to the agreed scope: you moved a wall, upgraded a finish, or added a window. Each should be priced and signed off before it is built.
The discipline matters more than the paperwork. A build where variations are agreed as they arise ends with a total you watched accumulate. A build where they are tallied up at the end ends with an argument.
Where the risk really sits
For most projects the genuine unknowns are few: the ground, the existing structure, approvals, and long-lead items. Everything else is either drawn or chosen. A builder who can tell you which of the four applies to your site, and what each would cost to go wrong, is giving you the real picture.
Reading a quote properly
Compare the assumptions, not just the totals. Check what has been excluded, what is an allowance, what is provisional, and which drawings were priced. The cheapest quote is often the one with the most left out, and you will pay the difference either way — the only question is whether you find out now or in month seven.
