Understanding Extension Costs:
7 Reasons Why Extension Quotes Vary (and How to Compare Them Properly)
"Why do my extension quotes vary so much?"
This is one of the most common questions we hear at Pristine Building Solutions. And it's a really good question.
According to HomeOwners Alliance, a standard 20 m² single-storey extension typically costs £40,000–£56,000 (excluding VAT) in the UK.
Several factors, including size, materials, location and scope, can explain the £16,000 price difference. But it can also be explained by the quality of the work, the builder’s experience, the actual cost of the project and the aftercare.
You might have a £10,000 or £20,000 difference in quotes and assume the most expensive builder is being greedy, but the truth is they’re just being transparent and are actually including everything the project needs (rather than sneaking extra costs in later).
The bottom line is that you should get several different quotes from builders when you’re thinking about adding an extension, but you shouldn’t always go for the cheapest quote. Instead, read on to find out why your extension quotes vary so much and how to work out which one truly offers you the best value.
7 Reasons Why Your Quotes Are So Different
If you’ve ever asked for quotes from builders for an extension, then you’ll know that one builder says £40,000, another comes in at £48,000 and the third at £56,000... For the same extension. So, why the £16,000 difference?
Let's be honest: not all quotes are created equal. Just because three builders have quoted on 'the same extension' doesn't mean they're actually pricing the same work. Here's why your quotes might be thousands of pounds apart, and what you need to look out for.
1. What’s included vs excluded
This is something we warn our potential clients about most when they ask why their quotes are so different. Two quotes might look similar on the surface, but one could include far more work than the other.
What often happens is the cheaper quote leaves out key elements that you'll end up paying for later, and sometimes at inflated rates because you're locked into that builder. Here are some common items that get excluded from cheaper quotes:
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Groundworks (foundations, drainage, excavation)
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Drainage alterations and connections
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Plastering and full decoration
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Flooring installation
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Kitchen supply (the quote might include fitting but not the actual kitchen units)
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Waste removal and skip hire
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Electrical work beyond first fix.
When these elements are excluded from quotes, it can add thousands to your final bill. What looked like a bargain at £40,000 (excluding VAT) suddenly becomes £52,000 (excluding VAT) once all the 'extras' are added in.
Knowing what is and isn’t included in your quote is invaluable when it comes to comparing them, which is why at Pristine Building Solutions, we're committed to being completely transparent about what's included and what isn't.
We'd rather you know upfront what to expect than be hit with nasty surprises halfway through the build. This honesty helps you budget properly and builds the trust that's essential for a successful project.
2. Allowances (Provisional Sums)
Provisional sums, or the estimated costs for things that haven't been fully specified yet, are another factor that can make builder quotes vary so much.
An example of a provisional sum is if you haven't chosen your tiles yet, so your builder pops in an allowance for this item.
However, one builder might allow £1,000 for bathroom tiles, while another allows £3,000. If the first builder's allowance is unrealistically low (maybe they've based it on budget tiles you'd never actually choose), your final bill will increase once you pick the tiles you want.
This is where experience really matters; at Pristine Building Solutions, we've done enough extensions to know what realistic allowances look like. We base our provisional sums on what actually gets chosen in real projects, not fantasy budget figures.
Our approach might make your initial quote slightly higher, but it means you won't get stung with surprise costs later. By including honest allowances, we make sure our clients can budget properly and avoid unexpected price hikes.
3. Vague Specs vs Detailed Breakdowns
Another reason why extension quotes can differ so much is the difference in the project breakdown. It’s often the cheaper builders that produce a vague (unhelpful) quote.
For example, "Build single-storey extension – £55,000."
This provides no actual details about the project or the source of the price estimate. There’s no breakdown of materials, labour, stages or what's actually included.
Now, here's an example of a detailed breakdown that shows you:
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Groundworks and foundations: £8,500 (excluding VAT)
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Brickwork and blockwork: £12,000 (excluding VAT)
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Roofing: £6,500 (excluding VAT)
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Windows and doors: £4,000 (excluding VAT)
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Plastering and decoration: £5,500 (excluding VAT).
By detailing what the project involves, you get a better understanding of where your money is going and what is included in the quote.
Vague quotes with fewer details make it easier to hide missing items, and they’re also impossible to compare with other quotes, making it hard to challenge costs if things go wrong, and leaving plenty of room for disputes about what was 'included'.
Unlike the vague quoting builders, we believe in clear, detailed quotes because you deserve to know exactly what you're paying for. To us, transparency matters because without all the information, you can’t make the right decision.
4. Quality of Work
Something that has a real impact on price is the quality of work. The phrase "You get what you pay for" comes to mind.
Cheaper quotes might seem attractive, but they involve rushed labour, less experienced tradespeople or corners being cut on things like foundations and insulation. You might not notice these shortcuts immediately, but they'll show up over time with the likes of damp issues, cold spots, cracks, or doors that don't hang properly.
Higher quotes often reflect skilled tradespeople, proper supervision and more time spent getting details right. At Pristine Building Solutions, we prioritise quality over quantity and would rather take on fewer jobs but do them properly than rush through multiple projects to compete on price alone.
That said, we're not perfect; mistakes can happen on any project. But what sets good builders apart is taking responsibility when things go wrong and putting them right. And that's part of the quality promise we make to every client – we will always come and fix any problems you have.
5. Materials and Finishes
Materials and finishes are another element that can drastically change your extension quote. If one builder opts for basic materials but another chooses high-spec, built-to-last materials, then the quotes will be very different.
Here are some examples of how different materials and finishes can vary in price:
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Brick types – Standard facing bricks cost £250 – £700 per 1,000 vs handmade or reclaimed bricks, which cost £800 – £1,500+ per 1,000.
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Roofing materials – Basic tiles cost around £20 – £35 per m² vs premium slate or zinc, which costs £65 – £100 per m²
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Insulation quality – Meeting minimum standards (approx. 270mm) can cost £300 – £500 for materials vs exceeding them (e.g., natural fibre or thick rigid boards which are usually over 300mm), may cost £1,000+
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Windows – UPVC windows cost around £250 – £600 per window vs aluminium, which costs £500 – £1,200+ per window vs hardwood, which can be up to £700 – £2,000+ per window
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Internal finishes – Basic plasterboard (i.e., standard 12.5mm plasterboard) usually costs between £9 and £13 per 2400mm x 1200mm sheet (£3–£5 per m²) vs soundproof boards (i.e., 12.5mm acoustic/soundproof board) generally costs between £15 and £25 per sheet (£5–£9 per m²).
The difference between these costs can easily add £8,000-£15,000 to an extension quote.
At Pristine Building Solutions, we always give honest advice on material choices, even if it means recommending something more expensive because it performs better in the long term.
Our focus is on value and longevity, not just the lowest upfront cost. We want to make sure you save money in the long run, so if there’s a high-quality material or finish that might suit you better, we will recommend it, but you have the final say.
6. Experience and Risk Allowance
That higher builder quote might be worth it if it means you’re paying for their experience and expertise. Extension projects aren’t always smooth sailing; they come with hidden risks that only experience can help you spot.
Some hidden extension risks can include:
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Unknown ground conditions (you might hit rock, old foundations or drainage)
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Structural complications when you open up walls
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Challenges of working around your existing building
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Coordination with neighbours and access issues.
Experienced builders price more realistically because they've seen these problems before and know what's likely to come up. Less experienced builders might underprice simply because they haven't encountered these issues yet, but halfway through the job, costs start to go up.
As experienced builders, we factor in real-world risks from the start. That might make our quote slightly higher initially, but it means far fewer surprise costs along the way.
7. Project Management, Compliance and Aftercare
The final reason why your extension quotes can differ so much is because of the behind-the-scenes elements that can increase the cost but are crucial to the success of the project.
For example:
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Proper project management and coordination
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Liaison with building control and inspectors
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Working with structural engineers when needed
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Running a clean, safe, respectful site
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Snagging and aftercare once the job is complete.
Some builders might not include these elements at all, which means their prices are lower.
Yes, these factors affect price, but they also significantly impact your homeowner experience. Living through an extension is disruptive enough without having to deal with chaos, mess or poor communication.
At Pristine Building Solutions, we take real pride in how we run our sites; we value professionalism and respect for your home.
When we’re working on a project, we treat your property the way we'd want ours treated, and this includes proper aftercare once the job is done, not just disappearing the moment we’re finished, because we’re reliable and respectful.
How to Compare Quotes Properly
Now you understand why quotes vary so much, we’ve put together a quick comparison checklist to help you determine which builder to choose.
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Your Comparison Checklist:
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Are all quotes itemised with proper breakdowns?
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Are provisional sums realistic based on the quality you actually want?
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What's specifically excluded? (Ask each builder to list exclusions clearly)
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Are materials and finishes properly specified?
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Has the builder explained their quote in person and answered your questions?
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Do you trust their communication style and feel confident working with them?
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Have they provided a portfolio or references so you can check their quality?
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What level of project management and site standards do they promise?
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Do they offer aftercare and ongoing support once the project is completed?
Determining which builder to choose is a big commitment, so don’t worry about taking your time and working out who is best suited for the project based on all of the above factors.
Choosing the Best Extension Quote
When you receive wildly different extension quotes, it usually means big differences in the scope (what's actually included), quality (the materials, workmanship and attention to detail) and transparency (how honest the builder is being about costs).
It’s worth remembering that the cheapest quote isn't always the best value. What matters even more than the price is finding a builder who's completely transparent about their pricing, delivers quality work and treats you and your home with genuine respect.
The right builder will take the time to explain their pricing clearly, answer all your questions and help you understand exactly what you're getting for your money. So, if you’re dealing with someone who doesn’t match this description or who has vague quotes... run!
Need honest advice on your extension?
At Pristine Building Solutions, we're always happy to talk through your project, even if you're still comparing options. We'd rather you have all the information and make the right decision before rushing into something that doesn't work out.
Get in touch with us now to find out more about your extension or to discover how we can help you.