How to Budget a Renovation in London: Setting Costs and Controlling Them

The most common reason renovation projects go over budget is not unforeseen problems. It is that the budget was never realistic in the first place. Costs were underestimated at the outset — by the client, by the contractor, or both — and by the time the true cost became clear, there was too much invested to stop.

This guide explains how to build a renovation budget that is actually accurate, what contingency you need and why, and how to protect your budget through the build.

Start with the right number

The first step is understanding what you are actually asking for. Many clients come to us with a project vision and a budget number they have arrived at by some combination of internet research, neighbour anecdote, and wishful thinking. The two are frequently incompatible.

Before you can build a budget, you need a brief. The brief needs to answer:

• —What is the scope of the work? (Full renovation, partial, extension, structural alterations?)
• —What is the specification level? (Bespoke joinery and natural stone, or mid-range fitted and porcelain tile?)
• —Are there planning constraints? (Listed building, conservation area, basement?)
• —What is the condition of the existing building? (Victorian terrace with unknown structure, or modern flat with known services?)

The brief and the budget need to be developed together. If you have £500,000 to spend, the brief needs to be sized to that budget. If you have a fixed brief, the budget needs to reflect what that brief actually costs.

Cost benchmarks for London renovation

The following are approximate benchmarks for high-specification London residential renovation. These are not quotes — actual costs depend on specification, location, condition, and market conditions. Use them to sense-check, not to plan.

For a more detailed breakdown of whole-house costs, see our London renovation cost guide.

These figures include all construction and fit-out work but typically exclude:

• —Professional fees (architect, structural engineer, party wall surveyor)
• —Planning application fees
• —Furniture, artwork, and soft furnishings
• —Appliances (where not part of the kitchen specification)
• —VAT (most residential renovation work attracts 20% VAT)

Professional fees for a major project run to 10–15% of the construction cost. Add VAT on top of everything. These are real numbers that need to be in your budget.

The contingency question

Every renovation budget needs a contingency. The question is how much.

10% is the absolute minimum for a straightforward project in known condition with no structural complexity. It is not enough for most London renovation projects.

15–20% is appropriate for most projects in older London properties — Victorian and Edwardian buildings regularly reveal unexpected conditions once you open up walls, floors, and ceilings. Previous poorly-executed works, buried services, structural modifications that were never properly engineered, and damp that didn't show up in the survey are all common.

20–25% is appropriate for listed buildings and complex structural projects where the risk of unforeseen conditions is higher and the remediation costs when they occur are larger.

This is not pessimism. It is arithmetic. Projects that are budgeted with no contingency do not come in on budget — they come in at whatever the actual cost is, and the client faces an unplanned decision mid-build.

Critically, the contingency is not for scope changes. It is for genuine unknowns. Scope changes — things you decide to add during the project — should be managed as deliberate decisions, not absorbed silently into contingency.

What to watch for in contractor quotes

When you receive tenders or quotes from contractors, the headline number is only the starting point. Look at:

What is included and excluded. A quote that excludes statutory approvals, temporary works, skip hire, and trade attendance for specialist subcontractors will inflate significantly by the time it reflects actual project cost. Ask for a fully inclusive price — labour, materials, plant, attendance, statutory fees — and list explicitly what is excluded.

The assumptions. A good tender is based on drawings and a specification. A quote produced from a brief conversation and a walk-around is not a price — it is an estimate, and it will change. The more detailed the documentation you provide at tender stage, the more accurate the responses will be.

The programme. A quote without a programme is incomplete. If the contractor cannot tell you how long the work will take, they do not yet understand the project. Programme matters for budgeting too — prolonged projects cost more in prelims (site management, welfare, scaffold, equipment hire).

The variation procedure. How are changes to scope instructed and priced? This is the most common source of overspend. If the contract does not specify a clear variation procedure, disputes over the cost of changes are almost inevitable.

How to control costs through the build

Getting to a good budget number is one problem. Keeping to it is another.

Freeze the design before work starts. The most expensive thing you can do is make design changes during the build. A wall that costs £800 to build costs £4,000 to move once it has been built, plastered, and painted. Decision changes are significantly more expensive than decisions made at the right time.

Manage variations formally. Agree that every variation — however small — is instructed in writing before it is carried out, with a cost and programme impact agreed in advance. This does not slow projects down. It prevents the common end-of-project conversation about why the final account is 30% higher than the original quote.

Review cost monthly. Your project manager should provide a monthly cost report showing budget, committed cost, forecast final account, and contingency remaining. If you are not receiving this, ask for it. Surprises at the end of a project are the result of not looking at the numbers during it.

Resist scope creep. The impulse to improve things while the builders are in — to do the loft while you are doing the ground floor, to add the roof terrace while you have scaffolding — is understandable. Sometimes it is the right decision. But each addition should be evaluated as a new decision: does this fit the budget, and is this the right time?

Professional fees — do not underestimate them

A common budget mistake is to plan carefully for construction costs and then discover that architect, engineer, and surveyor fees add another £80,000–£150,000 to the bill.

For a whole-house renovation with extensions and planning applications, professional fees typically break down as:

• —Architect: 8–12% of construction cost
• —Structural engineer: 1–2% of construction cost
• —Party wall surveyor: £2,000–£8,000 (depending on number of adjoining owners and complexity)
• —Planning application fees: £206–£462 for most householder applications; more for larger schemes
• —Building control: £1,000–£5,000 depending on scope

If you are using a project manager (as opposed to an architect acting as contract administrator), add a further 3–5% of construction cost.

How we approach budgeting at ASAAN

At ASAAN, we provide cost guidance at the earliest stages of a project — before design work has been commissioned, if a client wants an honest view of what a scheme is likely to cost. We would rather have that conversation at the start than discover mid-project that the budget and the brief are incompatible.

Our project managers track cost and programme rigorously throughout the build, and clients receive monthly cost reports as standard. We do not operate on the basis that problems will resolve themselves.

If you would like to discuss your project and get an early view on budget, contact us to arrange a consultation. You can also review our portfolio for examples of the scale and specification of projects we deliver.