Why Do Extension Quotes Vary So Much Between Builders?

You've done everything right. You've had three builders round, they've all looked at the same plans, and you've ended up with three quotes that are thousands of pounds apart. It's baffling — and in some cases, the gap between the cheapest and most expensive quote runs to tens of thousands.

This is one of the most common concerns we hear from homeowners across Oxfordshire, and it's a completely legitimate one. Understanding why quotes vary so dramatically is the key to making a good decision — because cheapest rarely means best value, and expensive doesn't automatically mean better.

They're Not Pricing the Same Thing

This is the most important point, and it's frequently overlooked. Two builders looking at the same drawings will often price very different scopes of work, because drawings don't capture every decision.

One quote might include full project management; another assumes the client manages their own trades. One might include a full structural steel package; another has priced provisional sums for steels to be confirmed later. One might include high-quality insulation to exceed building regulations standards; another has priced to minimum compliance.

Before comparing prices, make sure you're comparing what's actually included.

Specification Differences

Material and finish specifications drive significant cost differences. A quote that includes timber windows will come in lower than one that includes aluminium or timber-aluminium composite. A quote pricing standard plasterboard will differ from one pricing specialist acoustic or fire-rated boarding.

Roofing materials — from basic felt and batten to high-specification membranes — vary considerably in cost. Insulation grade, brick specification, and floor finishes all make a difference. If the specification isn't nailed down before quotes are obtained, each builder will make their own assumptions — and those assumptions will diverge.

Overheads and Running Costs

Established, well-run building companies carry higher overheads than sole traders or small operations. They employ site managers, carry comprehensive insurance, run vehicles and equipment, and invest in systems that keep projects on track. These overheads are reflected in their pricing.

A sole trader working from a van with minimal insurance can underprice an established contractor significantly. That doesn't make them better value — it may simply mean less protection, less management, and more risk for you if something goes wrong.

Labour Quality and Trade Rates

The people doing the work matter enormously to the final result — and quality trades cost more. A builder who uses well-paid, skilled, time-served tradespeople will price higher than one who relies on cheaper labour.

This shows up most visibly in the finishing trades. Plastering, joinery, and tiling in particular are areas where the difference between competent and excellent is immediately apparent, and where cutting corners on labour quality has lasting consequences.

The lowest quote is often low because of cheaper labour, not because the builder has found a smarter way to do the work.

Contingency and Allowances

A thorough, honest quote will include reasonable allowances for items that can't be fully quantified at tender stage — particularly groundworks and foundations, where unforeseen conditions are a genuine risk. Older properties in areas like Radley, Botley, Steventon, and parts of central Oxford can carry additional uncertainty around drainage layouts and original foundations.

A builder who has properly priced these risks will come in higher than one who has simply not included them. The danger of a quote with no contingency or provisional sums is that those costs don't disappear — they arrive as variations during the build, turning an initially attractive price into something far higher by completion.

How to Compare Quotes Properly

Ask each builder to break their quote down clearly, so you can see what's included and excluded. Key questions to ask:

Does the price include project management and site management, or just labour and materials?

What specification of windows, insulation, and roofing has been priced?

Are structural steels and groundworks firm prices or provisional sums?

What are the payment terms, and are they staged against progress?

What accreditations and insurance does the company hold?

A builder who welcomes these questions and answers them clearly is demonstrating the kind of transparency you want on a project worth tens of thousands of pounds. One who can't — or won't — explain their pricing in detail is a risk.

What We Include at Acute Homes

Our quotes are detailed and transparent. We price to a clearly defined specification, include full project management, and explain any provisional sums clearly so you know what's firm and what may vary. As TrustMark approved contractors and Which Trusted Trader members for over 10 years, we stand behind our pricing and our work.