15–30% Quote Surprises: Extension Quote Checklist for UK Homeowners
Use an itemised, VAT-inclusive checklist and spreadsheet method to normalise bids, spot 15–30% outliers, and get an instant, itemised quote from The...
15–30% Quote Surprises: Extension Quote Checklist for UK Homeowners
A credible extension quote is itemised by trade and phase, names materials and specifications, states its VAT position clearly, and flags every prime cost and provisional sum in pounds rather than percentages. It lists exclusions in full, sets out a payment schedule tied to build milestones, and comes from a contractor who can produce insurance certificates without hesitation. Send the same written brief and drawings to every bidder, then insist on itemised, written quotes you can put side by side.
TL;DR:
- A detailed extension quote should itemize costs by phase, including preliminaries, groundworks, structure, roofing, finishes, and professional fees, to allow proper comparison.
- Comparing quotes requires using the same brief and drawings, normalizing figures for VAT, and including allowances for exclusions and provisional sums to reveal true cost differences.
- Prime cost and provisional sums should be clearly specified in pounds within the total, with limits around 15 percent of the overall scope to avoid hiding uncertainties.
- Homeowners should verify insurance certificates, demand proper contracts, set milestone-based payments, and clarify who handles building control for legal and financial protection.
- Using an instant online quoting service offers comprehensive, itemized, VAT-inclusive prices with live 3D models, reducing uncertainty and speeding up the decision process.
Table of Contents
- What should a good extension quote actually include?
- How do you compare extension quotes side by side?
- What are PC sums, provisional sums and VAT doing to your total?
- What insurance and contract terms should you insist on?
- What does a single-storey extension actually cost per square metre?
- What questions should you ask every builder before signing?
- How The Extension Works removes the guesswork from quoting
- Price versus certainty: which should win?
- Get an instant, itemised quote before you commission anyone
- Sources
- FAQ
What should a good extension quote actually include?
Most homeowners can spot a badly written quote once they know what to look for. The problem is that a lot of quotes look professional on the surface, with a logo and a total at the bottom, while hiding the detail that actually determines what you’ll pay. A genuine line-by-line breakdown protects you from that.
Here’s what a properly detailed quote should show, broken down by phase:
- Preliminaries: scaffolding hire and erection, site welfare (toilets, storage), site protection for existing rooms and gardens, and skip hire with the number of skips assumed.
- Groundworks and foundations: the assumed excavation depth, soil type, and what happens to spoil, plus any drainage or soakaway allowance. This is one of the biggest sources of later disputes, because ground conditions are rarely certain until digging starts.
- Structure and steels: beams, lintels, padstones, and who’s supplying and fitting them. If steels are priced as a provisional sum, ask why, since a competent structural calculation should let a builder price this properly before you sign anything.
- Roof, glazing and named materials: either a specific product and brand, or a clearly written PC sum with the assumption stated (for example, “bifold doors, PC sum £3,500 supply only”).
- First fix, second fix and finishes: kept as separate lines, not bundled into one number, so you can see where electrics, plumbing, plastering and decorating sit.
- Professional fees and statutory charges: building control fees, party wall surveyor costs if relevant, and any planning-related charges listed separately from the build cost.
- Exclusions and validity window: a dated list of what’s not included, and how long the quote remains valid before prices might change.
A quote missing three or more of these isn’t necessarily dishonest, but it’s incomplete, and an incomplete quote is one you cannot properly compare against anyone else’s.
How do you compare extension quotes side by side?
Comparing quotes only works when every builder has priced the same job. That sounds obvious, but it’s the step most homeowners skip, sending vague verbal briefs to different builders and then wondering why the numbers don’t line up.
- Issue an identical brief and drawings to every bidder before asking for prices. If you don’t have proper planning drawings yet, get them done first. A builder pricing from a sketch and a builder pricing from a full spec are answering different questions.
- Build a comparison spreadsheet: one row per work package (groundworks, structure, roof, glazing, first fix, second fix, finishes, preliminaries), one column per quote. Force every cell to contain a number, “included,” or “excluded.” Never leave a cell blank.
- Normalise every figure to a VAT-inclusive total. Some contractors quote net, others quote gross, and mixing the two makes comparison meaningless.
- Price exclusions and provisional sums back into the headline total so each column represents the same actual scope of work, not just the same paperwork.
- Read the spread once everything is normalised. Quotes for identical scope commonly spread by 15 to 30 percent due to differing allowances and site assumptions, and that clustering itself tells you roughly where the market rate sits.
Pro Tip: If one quote sits more than 15 to 20 percent below the rest after normalisation, don’t celebrate. Ask exactly what’s been left out or under specified, because a genuinely cheaper builder using the same materials at the same margin is rare. A cheaper number usually means a thinner spec.
The cheapest headline figure is often the cheapest precisely because it excludes items like scaffolding or skip hire that every other bidder included. Once you add those back in, the “bargain” quote frequently lands in the same range as everyone else’s, or higher.
What are PC sums, provisional sums and VAT doing to your total?
These three things cause more quote confusion than anything else in an extension project, mostly because builders use the terms inconsistently and homeowners rarely ask for clarification.
A prime cost (PC) sum is an allowance for a named item that hasn’t been chosen yet, a kitchen worktop, a bathroom suite, external doors. A provisional sum covers work that can’t be fully specified until it starts, most often groundworks where the ground conditions are unknown until someone digs a trial hole.
Both are legitimate tools when used properly. The trouble starts when they’re used to hide uncertainty rather than manage it.
- Check whether PC and provisional sums sit inside or outside the headline total, since a “£95,000” quote with £15,000 of provisional sums excluded is really a £110,000 job.
- Insist on pounds, not percentages. “Provisional sum for groundworks” tells you nothing; “provisional sum for groundworks, £8,000, based on 600mm strip foundation depth” tells you exactly what’s assumed.
- Set yourself a red line: if provisional and PC sums together exceed roughly 15 percent of the total, treat the whole document as an estimate rather than a quote, and ask for named suppliers or capped figures before proceeding.
- On VAT, confirm whether the contractor is VAT-registered and whether the figure quoted already includes it. A non-registered small builder and a VAT-registered firm can look identically priced until you add 20 percent to one of them.
A well-specified quote will also include a short written note on ground-condition assumptions wherever foundations carry a provisional sum, covering depth, soil type and how spoil gets removed from site, which you can verify using a detailed Property Condition Report.
What insurance and contract terms should you insist on?
Price matters, but the paperwork around the price is what actually protects you if something goes wrong mid-build. This is the section most homeowners skim past, which is exactly why it causes the most expensive problems later.
- Ask to see insurance certificates directly, not just a mention of “fully insured” on a website. You want public liability cover and contract works insurance, and you want to check the expiry dates yourself rather than taking someone’s word for it.
- Require a proper written contract, not just the quote. Essentials include a reference to the agreed drawings and specification, a clear variation procedure for changes mid-build, defined milestones, a retention arrangement, and a route for resolving disputes. The JCT Home Owner Contract is a sensible template for domestic extension work of this scale.
- Set payment terms that match milestones, not the calendar. A reasonable deposit, followed by staged payments released against completed work, with a retention of around 2.5 to 5 percent held back until snagging is resolved. Avoid any arrangement built around a large upfront cash deposit.
- Confirm who’s arranging building control sign-off and whether that fee sits inside or outside the quoted price. It’s a small line item that gets forgotten surprisingly often.
None of this is exotic. It’s the standard homeowner protection that any properly written quote should lead into as a matter of course.
What does a single-storey extension actually cost per square metre?
Rough per-square-metre figures move around depending on region and spec, so treat any number here as context for sense-checking a quote, not a fixed price to hold anyone to.
- The biggest cost drivers are usually glazing, steels, groundworks and preliminaries, in roughly that order of impact on the headline figure.
- Bifold or sliding glazed doors can shift the budget considerably depending on the system chosen, so check exactly which product a quote’s PC sum for glazing assumes.
- VAT, provisional sums, and commonly excluded items like scaffold hire, skip hire and making good to existing rooms all add to the headline price, sometimes by a meaningful margin once totalled.
- Build in a contingency buffer on top of the quoted total. Extensions involving groundworks or older properties carry more uncertainty than a straightforward like-for-like rebuild, and a buffer stops a provisional sum landing badly midway through the job.
What questions should you ask every builder before signing?
Ask these seven questions of every bidder, in writing, and compare the answers as carefully as the price itself:
- What have you assumed in this quote that isn’t stated explicitly?
- What’s excluded, and why?
- Which items are provisional, and what’s the cap on each?
- Who arranges building control sign-off, and is that fee included?
- How will variations mid-build be priced and agreed?
- What exactly triggers each payment on the schedule?
- What paperwork do I receive at completion, including warranties?
Watch for the usual warning signs alongside the answers. An unexplained price sitting 25 percent or more below every other quote, requests for cash-only payment, reluctance to provide insurance certificates, no written programme of works, or pressure to sign immediately are all reasons to walk away rather than negotiate.
Get at least three quotes before deciding anything, and consider more if the numbers you receive don’t cluster reasonably close together once normalised.
How The Extension Works removes the guesswork from quoting
Most of the friction in this process comes from waiting weeks for a quote, then discovering it’s vague once it arrives. The process is designed to skip that step entirely, combining design, planning and build into one online journey with instant quoting and live 3D modelling, so you see an itemised, VAT-inclusive price and a visual model of your extension before committing to anything.
Fixed pricing and a dedicated project team remove much of the provisional-sum uncertainty described above, since scope is agreed upfront rather than discovered mid-build. Every project also carries a workmanship warranty. If you want to check the professional-fee side of a quote independently, the guides on structural engineer fees and planning fees are worth reading alongside whatever quote lands in your inbox.
Price versus certainty: which should win?
Homeowners fixate on the bottom line more than any other number on a quote, and it’s the wrong instinct. Completeness of scope matters more than the headline figure, because an incomplete quote is a promise with gaps in it, and those gaps get filled later at a price you didn’t negotiate.
Keep every quote, every assumption and every email in writing, and don’t sign anything until the quote you like has been converted into a proper contract that references your actual drawings. A quote that’s £3,000 higher but names every material and caps every allowance will usually cost you less than the cheaper one, once the provisional sums start landing.
— Esskay
Get an instant, itemised quote before you commission anyone
A service alternative to weeks of waiting for written quotes to trickle in is to answer a short online brief, and receive an instant itemised quote alongside a live 3D model of your extension, with the price shown VAT-inclusive from the outset.
That means no chasing five different builders for five differently formatted documents, no guessing which provisional sums are hiding inside a headline number, and no separate calls to check VAT status. Everything from the initial design through to a dedicated project team and a workmanship warranty runs through one process, so the number you see is close to the number you’ll actually pay. If you’d like to see finished results before deciding, the extension gallery shows completed projects at a similar spec level to what’s typically quoted.
Use the checklist above on any quote you already have, then get your own instant extension quote to see how it compares.
Sources
For deeper checks, see Gov, HSE site safety rules, and the guide to provisional sums.
FAQ
Is £30,000 enough for an extension?
A small, simple single-storey extension with a basic specification has a lower cost, but larger extensions or those with significant glazing, steels or groundworks generally run well above this once VAT and provisional sums are included.
What is the 3 metre rule for extensions?
Under permitted development rules, a single-storey rear extension on an attached house can generally extend up to 3 metres from the original rear wall without planning permission, rising to 4 metres for a detached house, subject to conditions set out in official planning guidance.
How much would a 20 square metre extension cost?
Costs vary widely by region, specification and site conditions, so there’s no single reliable figure. The most accurate way to budget is to get an itemised, VAT-inclusive quote for your specific brief and drawings rather than relying on a generic per-square-metre estimate.
What is the 50% rule for house extensions?
The “50% rule” generally refers to permitted development limits, where extensions and additions must not cover more than half of the land around the original house. Always confirm the specific limit for your property using GOV.UK’s planning guidance or your local planning authority.
How many quotes should I get for an extension?
At least three written, itemised quotes based on the same brief and drawings is the practical minimum, with more sought if the figures don’t cluster within a reasonable range once normalised for VAT and exclusions.