Getting three quotes and finding they differ by £20,000 or more is common, and it’s one of the most confusing parts of starting a building project. The variation rarely means one builder is trying to overcharge you or another is cutting corners — though that does happen. More often, it means the quotes are not pricing the same thing. Understanding why makes it possible to compare them properly, and to make a decision based on value rather than just the bottom-line figure.

Key takeaways

• Large differences between quotes usually reflect different scopes, not different margins
• What’s included and excluded matters more than the total price
• Provisional sums are estimates for unknowns — they need to be clearly identified and consistently handled across quotes
• A low quote often means less has been included, not that the work can genuinely be done for less
• The right question is not “which is cheapest?” but “which is pricing what I actually need?”

Why quotes for the same project can differ significantly

When a builder produces a quote, they’re working from their interpretation of the brief, their assumptions about specification, and their judgement about how long the work will take. If you haven’t provided a detailed specification of works — and most homeowners haven’t at the early quote stage — each builder is effectively pricing a slightly different version of your project.

That gap in scope is where most of the variation comes from.

A single-storey extension might be quoted at £85,000 by one builder and £110,000 by another. The difference could reflect entirely different assumptions about what the job involves: whether groundworks include dealing with a suspected drainage run, whether the quote allows for structural engineer fees, whether the kitchen fit-out is included, whether VAT is added. None of those differences show up in the headline figure — they show up when you read the detail.

The Royal Institution of Chartered Surveyors recommends getting at least three detailed, itemised quotes for any significant building work, specifically because it’s only when you compare line by line that the scope differences become visible. A lump-sum quote with no breakdown makes this comparison almost impossible.

What’s included and what isn’t

The single most important thing to check in any quote is what’s explicitly excluded. Most builders list exclusions somewhere — often in small print, sometimes not at all on informal quotes. Common exclusions include:

VAT. Extension and renovation work is subject to 20% VAT. Many builders quote ex-VAT, which adds significantly to the headline figure. A quote of £90,000 ex-VAT and a quote of £100,000 including VAT are closer than they appear. Always check whether figures are inclusive or exclusive of VAT before comparing them.

Professional fees. Architect fees, structural engineer costs, planning application fees, and building control charges are regularly excluded from builder quotes. These can add 10–15% to the total project cost. If one quote includes them and another doesn’t, the comparison is meaningless until you adjust for this.

Groundworks beyond standard. Most quotes assume straightforward ground conditions. If the site has suspected drainage runs, a high water table, poor load-bearing soil, or proximity to trees, additional groundworks may be needed — and these are often either excluded or covered by a provisional sum. Knowing which builder has priced these in, and which hasn’t, matters.

Fit-out and finishes. Kitchens, bathrooms, flooring, decorating, and external works are routinely excluded from extension quotes unless specifically stated. A quote that says “extension to shell and plaster” and one that says “extension including kitchen fit-out and flooring” are not comparable.

External works. New patio, path, drainage connections, landscaping — these often fall outside the building contract entirely. If your project involves these elements, check whether any quote includes them.

Provisional sums: what they are and why they matter

A provisional sum (sometimes written as PS or PC sum — the latter standing for prime cost) is a budget allowance included in a quote for work or materials whose cost can’t be fixed at the time of quoting. Common examples include:

• Groundworks, where the actual scope depends on what’s found when the ground is opened up
• Structural steelwork, where the engineer’s design isn’t yet finalised
• Kitchen or bathroom fittings, where the client hasn’t yet chosen products
• Any specialist work that requires a subcontractor quote the main contractor hasn’t yet obtained

Provisional sums are a normal and necessary part of quoting on projects that aren’t fully defined. The problem arises when they’re used inconsistently across quotes — or when they’re not clearly identified at all.

A quote with a low provisional sum for groundworks isn’t necessarily cheaper than one with a higher allowance. It may simply mean the builder has underestimated the likely cost, or has deliberately priced low to win the work, knowing the provisional sum will increase once the ground is opened. This is one of the most common ways that projects end up costing significantly more than the original quote.

When comparing quotes, list every provisional sum in each quote, note the figure allowed, and check whether the assumptions behind it are comparable. If one builder has allowed £5,000 for groundworks and another £15,000 for the same site, that gap needs to be explained — not just averaged.

Quality and specification assumptions

Even when two quotes cover the same scope, they may assume different specification levels. Timber grade, insulation thickness, window quality, the number of fixing points allowed for tiling, the depth of preparation before floor finishes — these details affect both cost and outcome.

A builder who prices for standard materials at standard rates and a builder who prices for better materials at a higher labour input will produce different figures, and both may be right for their respective assumptions. The question is which set of assumptions matches what you actually want.

The most reliable way to equalise this is to provide a specification alongside your brief — or to ask each builder to state their material and product assumptions within the quote. If they’re both pricing for equivalent quality, any remaining difference comes down to labour rates, margin, and programme.

Labour time and programme

Builders price time differently. A larger firm with more overhead may allow more labour hours but also bring more resource to the project, completing it faster and with less disruption. A smaller operation may price fewer hours but take longer. Neither is necessarily wrong — the right answer depends on what matters more to you: cost or speed.

What’s worth checking is whether the programme in each quote is realistic. A very fast programme at a low price can indicate either genuine efficiency or an unrealistic allowance that will slip. If you have a deadline — moving in before a school term, completing before winter — a realistic programme may matter more than the cheapest figure.

How to compare quotes properly

Once you have quotes back, treat the comparison as a structured exercise rather than a straight price check.

Set the total cost on a like-for-like basis first. Add VAT if it’s been excluded. Add professional fees if they’re not in the quote. Add any elements that are in one quote but missing from another. You’re trying to get to an equivalent total for an equivalent scope before you compare a single figure.

Then go line by line through the key cost categories — groundworks, structure, roofing, first fix, second fix, fit-out, external works — and check what each builder has allowed. Where the figures differ significantly, ask why. Sometimes the answer is a different approach to the work; sometimes it’s a different assumption about scope; sometimes it’s a provisional sum that needs discussion.

If a quote is significantly lower than the others and the scope appears equivalent, ask the builder to walk you through it. The explanation will usually reveal whether it’s genuine efficiency, a low provisional sum, or something that’s been missed. A reputable builder will have no difficulty doing this — and their willingness to do so is itself a useful signal.

Frequently asked questions

Why is one builder’s quote much lower than the others?

Most commonly because less has been included. Check VAT, professional fees, provisional sums, and fit-out items against the higher quotes. If the scope is genuinely equivalent and one builder is significantly cheaper, ask them to walk through the difference — it’s a reasonable question and a reputable builder will welcome it.

Should I always go with the middle quote?

Not necessarily. The middle quote is a useful starting point if you’re uncertain, but the right choice depends on scope, programme, the builder’s track record, and how clearly they’ve communicated what’s included. A higher quote that’s fully itemised and realistic may represent better value than a lower one with significant provisional sums or vague exclusions.

What is a provisional sum in a builder’s quote?

A provisional sum is a budget allowance for work or materials that can’t be priced precisely at the time of quoting — typically groundworks, structural elements, or client-supplied items not yet specified. It’s a placeholder, not a fixed price. When comparing quotes, check that provisional sums are clearly identified and that the allowances are comparable across quotes.

Is VAT always added on top of a builder’s quote?

Most builders quote ex-VAT on residential projects, meaning 20% is added on top. Some include VAT in the headline figure. Always check which applies before comparing quotes. New builds attract zero-rated VAT; renovation and extension work is standard-rated at 20%.

How many quotes should I get?

Three is the standard recommendation for most projects. This gives you enough to identify whether one quote is an outlier in either direction, and to triangulate what a realistic cost looks like. Getting more than three on a complex project rarely adds value — it adds time, and it asks builders to invest time in quotes that most of them won’t win.