Contingency Budget for an Extension: How Much to Set Aside

For most extensions, hold a contingency of 10% of the build cost on top of your quote. Raise it to 15% or more if the house is old, the work touches the structure or the quote is vague. Keep the money in your own account, separate from the builder’s price, and release it only against written, agreed changes.

This guide explains what a contingency is, how it differs from a fixed-price quote, how to size it for your own project and how to manage it so it protects you rather than quietly disappearing.

TL;DR

• A contingency is your own reserve for costs nobody could price in advance. It sits on top of the contractor’s quote.
• Published UK guidance starts at 10% and rises to 15% or more for older homes, structural work or thin quotes.
• A fixed price reduces uncertainty, but your own changes of mind and unknown conditions still need a reserve.
• Hold the reserve separately and release it only against a written variation or evidenced overrun.
• If you do not need it, it is still your money.

Table of Contents

• What a contingency budget is, and what it is not
• How much contingency for an extension
• What raises the figure
• What an extension contingency should cover
• How a fixed price changes the contingency
• Where to keep it and how to spend it
• Our perspective
• Get a fixed price before you set your reserve
• FAQ
• Sources

What a contingency budget is, and what it is not

A contingency budget is a sum you set aside, over and above the agreed build price, to pay for things that could not be identified or priced when the contract was signed. It is not a discount you hope to get, it is not padding in the builder’s quote, and it is not a fund for upgrading the kitchen halfway through.

The quote answers the question “what will it cost to build what we have drawn?” The contingency answers “what if reality differs from the drawings?” Those are different risks, held by different people, so they belong on different lines.

The professional cost-planning standard in the UK, RICS NRM 1, treats this as a risk allowance and splits it into design development risks, construction risks (including ground conditions, existing services and delays by statutory undertakers), employer change risks and other employer risks. It also says transfers into and out of those allowances should be recorded at each stage. You do not need a quantity surveyor to borrow the idea: write down what could go wrong, and write down what you spend.

How much contingency for an extension

The short answer from the published UK guidance is 10% as a floor, and more when risk is higher. The sources differ slightly, which is useful: it shows the range rather than a single magic number.

A worked example makes it concrete. Suppose your contractor’s price for a rear extension is £60,000. At 10% your reserve is £6,000, and at 15% it is £9,000. That is a sum you hold in addition to the £60,000, not part of it. The figures are illustrative, not a quote.

If the lower end of the range (5%) feels tempting, remember what you are trading. The CIOB notes that if costs spiral beyond your contingency you may be forced to compromise on scope or quality of finishes. A reserve that is too thin shows up at the end of the job, when the flooring and decorating are still to pay for.

Why a percentage is only a starting point

RICS makes a point worth knowing: risk allowances should not be a standard percentage but a considered assessment of the risk, including how complete the design is and how much site investigation has been done. At the earliest stages the allowance can be a significant share of the estimated cost, and once the work is complete it falls to zero.

In practice that means your percentage should shrink as uncertainty shrinks. The more of the following you have in hand before you sign, the nearer to 10% you can sit:

• Full drawings, including structural calculations, agreed before the price is fixed.
• Knowledge of what is under the ground where the extension will sit, such as drains and manholes.
• A written specification for finishes rather than allowances to be decided later.
• No provisional sums, or very few.

The more of those are missing, the more reason to move towards 15%.

What raises the figure

Published guidance names several triggers. Consider each against your own house.

Age of the property. The FMB says unexpected issues can arise in any project, particularly in older properties, where hidden structural problems, outdated wiring or drainage issues may only become apparent once work begins. A pre-war semi in Sutton or Morden is the classic case.

Structural alterations. Taking out a load-bearing wall to open the back of the house onto the new space is common. The FMB suggests considering 15% or more where structural alterations are involved.

Ground conditions. HomeBuilding magazine’s guidance, as summarised by Parallel Architecture, puts the figure at 10% on most sites and 20% on a sloping site, bad ground or where there is a basement. Rear extensions on London clay are rarely dramatic, but drains running across the footprint are a frequent surprise.

A vague quote. Chatto Your Builder points to thin detail in the quote as a reason for about 15%. A one-page price with allowances in place of specifications leaves more to be settled later, and later is when it costs more.

What an extension contingency should cover

Use the reserve for genuine unknowns:

• Hidden defects found once walls, floors or ceilings are opened up.
• Drainage that needs diverting or repairing.
• Wiring or plumbing that has to be brought up to standard to connect the new work.
• Ground that needs deeper or different foundations than assumed.
• Overruns on provisional sums, supported by evidence.

The JCT’s guidance for its minor works contract defines a provisional sum as a sum for work that may or may not be carried out, or that cannot be accurately specified in the contract documents. If your quote carries several, each one is an open question, and your reserve is what answers it.

Do not use it for your own changes of mind. Parallel Architecture is blunt that a homeowner’s second thoughts are not unforeseen, and should be budgeted for separately. If you already suspect you will want a different kitchen layout or underfloor heating, add that to the budget as a planned item, and keep the contingency for the things you could not have planned.

How a fixed price changes the contingency

A fixed price shifts risk, but it does not abolish it. Understanding exactly where it stops is what lets you size your reserve sensibly.

The JCT guidance describes the contract price as the lump sum stated in the contract plus any VAT properly chargeable. It also says the precise sum can go up or down depending on changes to the work, the value of work instructed against provisional sums, and changes in levies and taxes for which the contractor is liable. A variation, in JCT terms, is a change to the work instructed on the employer’s behalf, which can be an addition or an omission.

RIBA adds a practical caution: a lump sum contract gives the best cost certainty, but needs full and detailed information at tender stage. That can be hard on domestic projects, especially where work is to an existing building with unknowns that cannot be established before work starts. The CIOB makes a related point: even with a fixed-price quote, the eventual contract may contain provisions to charge more where doing so is fair.

So the question to ask of any fixed price is not “is it fixed?” but “what is excluded, what is provisional, and what happens if I change something?” The more complete the design and specification behind the price, the fewer open items you carry. We explain how provisional sums work, and how to avoid them, in our guide to fixed-price rear extensions.

Planning route also shapes how much is settled up front. Knowing whether your project can proceed under permitted development or needs a fuller application removes one source of change, and this overview of permitted development rules is a sensible place to check what you can build without planning permission before you finalise a budget.

Where to keep it and how to spend it

The mechanics matter as much as the percentage. Money that is mixed into the project figure gets spent.

1. Hold it separately. Parallel Architecture advises keeping contingency outside the contractor’s price and outside the figure you tell yourself the project costs. Chatto Your Builder is explicit that it sits with the homeowner, not as a loose line the builder can draw on.
2. Decide who can authorise spending. Agree in advance, in writing, that the answer is you, and how each use will be recorded.
3. Release against paper. Pay out of it only for written variations you have agreed, or genuine provisional-sum overruns supported by evidence.
4. Track it weekly. The CIOB recommends a detailed cost tracker updated weekly and regular site meetings so problems are solved quickly. A simple spreadsheet with columns for item, reason, amount, date agreed and balance is enough.
5. Re-assess at milestones. As each stage completes, uncertainty falls. If the groundworks are finished and nothing turned up, your remaining risk is lower, and you can decide what to do with the unspent balance.

The CIOB also notes that an experienced builder or project manager helps keep costs under control, particularly under a fixed-price contract. Parallel Architecture frames the payoff well: if the contingency is not needed it is money you still have, but if it was never there, the project stops or the finish suffers.

A simple tracker

The figures are invented for illustration. The habit is what counts: every line has a reason, a number and an agreement.

Our perspective

We build one kind of project, the single-storey rear extension, and the biggest source of budget stress we see is not the build itself. It is ambiguity before the build: a price that depends on allowances, drawings that are not finished, and a planning route that is still uncertain.

That is why we put design, planning, structural calculations, project management and construction under one roof, and why we publish a fixed, VAT-inclusive price rather than a range. The fewer open questions at the point of signing, the smaller the reserve you need to hold, and the more of your money goes into the space you actually wanted.

We still recommend that every homeowner keeps a personal reserve for their own changes and for the things no one can see behind a wall. A fixed price and a reserve do different jobs. Our job is to make the first as complete as we can, and to back our workmanship with a 12-month warranty. Yours is to keep the second in your own account.

Esskay

Get a fixed price before you set your reserve

You can only size a contingency sensibly once you know what the main price covers. With The Extension Works you can design and price your rear extension online, see it in live 3D and get a fixed, VAT-inclusive price without a sales visit. Design, planning and construction are handled in-house by a dedicated team.

Booking is secured with a one-off booking fee of £499. If you want to see how the costs are built up before you start, our extension cost breakdown walks through the main elements.

FAQ

Is 10% contingency enough for a house extension?

For a typical, well-designed extension on a sound modern house, 10% is the figure most UK guidance starts from, including the FMB’s. For older properties, structural changes or a quote with little detail, the same guidance points to 15% or more.

Does a fixed-price quote mean I do not need a contingency?

A fixed price covers the work as specified, but variations, provisional sums and your own changes can still alter the final sum, as the JCT guidance describes. A reserve in your own account covers those items, so you are never forced to cut the finish to pay for them.

Should the builder know how much contingency I have?

It is best kept as your own line, outside the contractor’s price. Parallel Architecture advises holding it separately and deciding in advance who can authorise spending it. Releasing it only against written, agreed variations keeps control with you.

What happens to contingency money I do not spend?

It stays yours. Many homeowners use the balance towards landscaping, furnishing or simply keep it. It is sensible to review the remaining reserve at each completed stage, as risk falls once the groundworks and structure are done.

Is contingency the same as a provisional sum?

No. A provisional sum is an amount inside the contract for work that cannot yet be specified. A contingency is your own reserve outside the contract, for the unforeseen. Provisional sums that overrun are one of the things your contingency can absorb.

Sources

• FMB: How to budget for a renovation and avoid going over budget
• RICS: New rules of measurement (NRM 1)
• CIOB: Success factors for home building projects
• Selfbuild Extend Renovate: What contingency do you need?
• Chatto Your Builder: How much contingency for an extension or renovation?
• Parallel Architecture: Contingency on a home extension or renovation
• JCT: MWD 2016 Guidance Notes
• RIBA: Choosing the right building contract for a small domestic project
• Permitted development rules overview

Recommended

• Planning fees for an extension: what England homeowners need to know
• Get a Fixed Price Rear Extension in England and Avoid Provisional Sums
• Discharging planning conditions for extensions: a homeowner’s guide
• Rear extension cost in 2026: a UK homeowner’s guide