What actually drives the cost of building work

The ground

Foundations are the single biggest source of variance on a domestic job, and the least visible. A standard trench fill on good ground is one price. Trees within influencing distance, made ground, a high water table, a drain in the way, or an old mine working underneath are all a different price and sometimes a different foundation type.

This is why we look at the ground before quoting rather than after. A contractor who quotes an extension without asking what is under it has either priced for the best case or left themselves room to come back to you.

Structure and spans

Removing a wall costs what the steel costs plus what holding the building up while it goes in costs. A 3-metre opening in a non-load-bearing partition and a 5-metre opening carrying the floor above are different jobs entirely.

Two things drive it: the span, and where the load goes when it arrives at the ends. A beam that lands on an existing pier is straightforward. A beam that needs a new padstone, a new pier, or its own foundation is not.

Glazing and roof

On a rear extension the glazing is frequently the largest single line in the quotation. The gap between a standard patio door and a large aluminium sliding system runs into five figures on its own, and it is entirely a specification choice.

Roof type matters similarly. A warm flat roof with a lantern, a pitched roof tied into the existing, and a flat roof with rooflights are three different costs before anyone talks about finishes.

Finish level

This is where the widest range sits and where quotations diverge most, because it is the easiest thing to leave vague. Sanitaryware, kitchen units, tiling, flooring, ironmongery and lighting can be specified across a range of several times over on the same footprint.

It is also the most controllable. A scope that fixes the shell and gives you a stated allowance for the finishes lets you decide later without reopening the contract.

Access and constraints

How material gets in and spoil gets out is a real cost. A terraced house with no side access means everything through the house or over the roof, and it changes the labour on every single delivery. A steep site, a shared drive, a narrow lane, a permit-only street, or a conservation area restriction on materials all price differently from the same job on an open plot.

Comparing quotations that are not comparable

Three quotations with a wide spread usually means three different scopes, not three different profit margins. Before comparing the totals, check they contain the same work.

What to check in each one

• Is the foundation type stated, or is it "assumed standard"?
• Are the structural steels priced, or subject to the engineer's design?
• Is building control included, and whose name is the application in?
• Are the finishes specified, or is there a stated allowance, or nothing at all?
• Is making good to the existing house included, and how far does it extend?
• Is scaffolding, skips, welfare and site clearance in the number?
• Is VAT stated, and is it in or out of the headline figure?
• What are the payment stages, and is anything due before work starts?

The cheapest quotation

A number well below the others is information. Sometimes it is a contractor with a quiet month. More often it is a scope that leaves things out, an optimistic foundation assumption, or a price that depends on variations later.

Ask what it excludes. A contractor who cannot answer that quickly has not thought about it, and it will be your problem in week six.