Can a builder increase the price after I have agreed it?

It depends on whether the price was a firm quote for defined work or just an estimate, and on whether the work is inside the scope you agreed. A firm quote for the agreed work is binding, and cannot simply be raised. But anything beyond that scope, whether you asked for it or the building forced it, is an extra. An extra should be priced and agreed with you before it is carried out, not added to the final bill unannounced.

What is the difference between a quote and an estimate?

A quote is a fixed price for clearly defined work, and it is binding. An estimate is a considered best guess at the likely cost, and it is not. The practical lesson is in how you use them. Get a quote given against a scope that is fully defined, not an estimate given against a vague one. A firm number is only firm if the work it covers is spelled out, so define the work first, then ask for the price.

Who pays if the builder finds something unexpected?

It depends on what it is. A genuine unforeseen condition, something no reasonable survey would have found, is usually the client's cost. Something a survey should have picked up is arguable, and turns on your contract and who was engaged to check. A provisional sum, the placeholder for work not yet defined, is always adjusted to the real cost. This is exactly why up-front surveys and a clear contract are worth the money.

How do I stop scope creep on my own project?

Decide the brief before work starts, and hold to it. Most scope creep is a run of small “while we are at it” additions, each one reasonable on its own, that together move the number a long way. Two habits keep it in check: resist changing the design once the price is set, and price any change you do want before you agree to it. That way every addition is a decision with a figure attached, not a surprise on the final bill.

Do I need a quantity surveyor for a home renovation?

For a larger or complex prime-London project, it is usually worth it. An independent cost plan and independent cost control, from a quantity surveyor or a construction manager, give you numbers that are not tied to the builder pricing the work, and that independence is the value. For a small, simple job it can be overkill, and a well-defined scope with a clear quote may be enough. The bigger the risk, the more the independent view earns its fee.

How to Avoid Renovation Cost Overruns in Prime London

Why most renovations go over budget

Most renovations go over budget, and only a small share come in under. In a 2026 survey of UK homeowners by Houzz, about 38 per cent went over their budget, about a third came in on budget, and only about 3 per cent came in under. That survey polls a self-selected panel of active renovators, so if anything it flatters the picture. A separate 2018 study by the insurer Hiscox found roughly two in five renovators overshot, by an average of about 20 per cent.

So going over budget is the normal outcome, not the exception, and not bad luck. The pattern is far too consistent for that. Large construction and infrastructure projects overrun the vast majority of the time, which tells you the problem is systemic, built into how projects are set up rather than particular to any one site.

Here is the part that matters. A renovation does not go over budget on site. It goes over budget the day someone signs a price against a design that is not finished. At that moment, the contractor is pricing assumptions, and every gap between the drawing and the finished house becomes an extra later. The overrun is engineered in at the start, when scope, drawings and risk are still open. Avoiding a renovation cost overrun is therefore about what you do before the build, which is what the rest of this article covers.

The causes, and the one that matters most

The causes of overruns are well known, and one sits above the rest: an incomplete design at the point you sign a price. Studies of construction projects consistently find that design change is the single biggest driver of cost overruns. If the drawings and the specification are not finished when the contract is signed, everything left undecided becomes a change, and every change has a cost.

The rest follow from the same root:

Provisional and prime cost sums. A provisional sum is an estimate for work not yet defined enough to price; a prime cost sum is an allowance for an item not yet chosen. Both are adjusted to the real cost at the final account, the settling-up at the end, so a low guess quietly becomes a higher bill.
Variations. Any change to the agreed scope is added to the account, whether you decide to do more while the walls are open or the building reveals something.
Unforeseen conditions. In older homes, damp, rot, failed structure, hidden services and asbestos are often found only once floors and walls are opened.
Quotes bought on price. A suspiciously low quote usually means something was left out. The Federation of Master Builders advises treating one with caution.
Thin contingency. Contingency is the reserve for the unexpected; set it too low, or spend it early, and nothing is left for the genuine surprise.
Programme slippage. The time-related site costs, called preliminaries, run at roughly 8 to 15 per cent of the contract value, so a job that runs long costs more even when the scope has not changed.

None of these is exotic, and each has a specific prevention. That is where the rest of this article goes.

Fix scope and price before you build

The single most effective way to avoid an overrun is to finish the design before you commit a price. The RIBA Plan of Work, the industry's standard project framework, is clear that the design should be substantially complete before the building contract is signed. This is often called a design freeze: the point after which the design does not keep changing. Price a finished design and there is far less left to become an extra.

Next, get a proper cost plan. Instead of a single lump-sum figure from a builder, which hides its assumptions inside one number, an independent quantity surveyor, a professional who measures and prices building work, prepares a cost plan that prices the job element by element under the RICS New Rules of Measurement. That exposes what is assumed and what is priced, so you can see the risk rather than discover it.

For a larger prime-London project, a Pre-Construction Services Agreement is the strongest tool of all. This first-stage agreement brings the contractor in early, to advise on how the job will be built, help price it and reduce its risks, before the main build contract is committed. Projects run this way tend to reach the final account with fewer variations and fewer claims. The honest caveat is that a two-stage price can start higher than a single competitive tender, and the early contractor gains some leverage, so the sub-contract work should still be tendered on an open book, meaning the real prices are shared and visible.

Finally, resolve the provisional and prime cost sums before you sign. Decide the detail behind each one so it can be priced firmly, or price it through the pre-construction stage. Every guess you turn into a committed number is one less line that can drift.

Cut the surprises: surveys, contingency, and the right contract

You can shrink the surprises a build springs on you, and three things do most of the work: surveys, contingency and the contract. Start with surveys. Commissioning structural, damp, asbestos and drainage surveys before you price turns hidden conditions into known works. What a survey finds can be priced into the job; what it misses becomes an extra later.

This matters most in prime central London, because the stock here is old. Roughly a third of England's homes were built before 1945 and a fifth before 1919, and prime central London skews far older again. In Victorian and Georgian houses, solid walls, original services and hidden defects tend to surface only on strip-out, so there is more to find. Party wall and basement work add further complexity, covered in the Basement Impact Assessment guide.

Next, hold honest contingency. Contingency is a reserve set aside for what the drawings cannot predict. Treated properly, it is ring-fenced, sized to the risk the surveys reveal, drawn deliberately, and returned to you if it is not needed. It is not fat to be negotiated out of the price. As a rough rule of thumb, people hold around 10 to 20 per cent for a well-understood property and up to about 25 per cent for an older one with concealed conditions. No professional body sets a fixed figure, and the right number depends on what your surveys find.

Then choose the contract with its risk allocation in mind. A fixed-price contract puts the overrun risk on the contractor, but is only truly fixed if the design is fixed. A cost-plus contract, where you pay the actual cost plus a fee, puts the risk on you. Construction management, where you employ the trades directly and pay a construction manager to coordinate them on an open book, trades a single headline number for real visibility and control. A newer standard contract, the Target Cost Contract, shares any overspend or saving between you and the contractor. There is no free choice, only deciding where the risk sits with your eyes open.

Keep control once the work starts

Cost certainty is maintained during the build, not just set at the start. The main tool is regular cost reporting on an open book. A monthly cost report should show the agreed contract sum, the changes approved so far, the forecast final account, the contingency status, and the cost still to come. Shared and reconciled monthly, drift is caught in weeks rather than discovered at the end, when it is too late to act.

The second habit is change control. Every change should be priced and agreed before the work on it starts. Standard building contracts allow exactly this: the contractor can quote for a variation, including its effect on the programme, so you can approve it, question it or drop it before committing. A change is then a decision you make with the number in front of you, not a surprise on the final bill. Most of the worst overrun stories are just a series of small changes never priced until it was too late.

None of this is exotic. It is ordinary project discipline: know the design before you price it, price the risk honestly, and watch the money as the work goes on. The projects that stay on budget are rarely lucky. They are the ones where the hard thinking was done before the first wall was touched.

The number everyone wants, and the one that holds

The honest way to price a renovation is to finish the design first and price the risk openly, not to name a firm number before the job is properly worked out. The trade often does the opposite, and there is a reason. Clients want certainty and a low headline figure, and a single tidy price against an unfinished design gives them both, right up until the extras start. The overrun is the predictable result of the very thing that won the job.

Any builder in prime central London, Myrmex included, is asked for a firm price before the design is done. The comfortable answer is to give one. The honest answer is to say that a price is only as reliable as the design and the risk work behind it, and to do that work first: freeze the scope, price it properly, hold real contingency, and keep the cost visible as the job runs. A number without that behind it is not certainty. It is a smaller figure now in exchange for a bigger one later.

None of this makes a renovation cheaper by itself. It makes the final figure look like the first one, which is what people actually want when they ask how to keep a project on budget. If you want to work through what your own project should really cost, that is the next step.

This is general information, not a cost plan or a contract recommendation. Every project, building and brief is different, and the right approach comes from the design, the surveys and a cost plan built around your specific job.