You've got three quotes. You've picked a builder. You know what the project is going to cost. Except you probably don't, because the builder's quote tells you what the builder charges. It doesn't tell you everything else you're going to spend.
These additional costs aren't the builder's problem to communicate. They're yours to anticipate. Most renovation budget failures aren't caused by the builder's bill being larger than quoted: they're caused by the homeowner failing to account for the full picture of costs from the start.
Costs Before Work Starts
Surveys. Depending on your property and project, you may need a structural survey (£500-£1,500), a damp survey (£200-£400), an asbestos survey (£200-£500 for a Type 2 management survey), or a drainage survey (£150-£350). You may not need all of these, but you should know whether you do before you brief your architect or builder. Discovering asbestos during demolition stops a project and adds significantly to cost.
Planning application fees. As of 2024, a householder planning application in England costs £258. Larger applications or prior approval applications vary. This seems minor, but planning agents or planning consultants who prepare applications typically charge £500-£2,000 on top of the application fee itself.
Party wall surveyor fees. If your project involves work within three metres of a neighbouring property's foundations, or work on a shared wall, you'll likely need to serve Party Wall Act notices. If a neighbour dissents, you each need a surveyor. Fees per surveyor typically run to £700-£1,500 for a straightforward case, and you may be responsible for paying both sides' fees.
Temporary accommodation. If the works make your home uninhabitable (as major renovations often do), you need somewhere to live. Short-term lets in most UK cities range from £1,500-£4,000 per month depending on size and location. For a six-month project, this alone can add £9,000-£24,000 to your total spend.
Storage. Your furniture and belongings need to go somewhere. Self-storage for the contents of a three-bedroom house typically costs £200-£400 per month. Over six months, that's £1,200-£2,400.
Costs That Emerge During the Build
Utility upgrades. New kitchens and extensions often require electrical consumer unit upgrades (£800-£2,000), new boiler or heating circuits, and occasionally new water supplies. These may be included in your main contractor's quote or may be excluded and separately quoted by a specialist. Clarify this in writing before you start.
Unforeseen structural issues. Older properties regularly conceal problems that only reveal themselves when walls are opened or floors lifted: concealed rot, incorrect previous alterations, failing lintels, undersized foundations. Your contingency should cover these, but you need to have the contingency in the first place.
Skip hire and waste disposal. Unless your contractor explicitly includes skip hire in their quote (and many don't for smaller works), you'll be paying separately. A standard skip hire in an urban area costs £250-£400 per week. For a kitchen or bathroom fit-out that's generating significant waste, several weeks' hire adds up.
Upgraded specifications mid-build. It's extremely common for homeowners to see something during the build that they hadn't anticipated and decide to upgrade: a better boiler, a larger rooflight, underfloor heating in a room that wasn't originally planned. These upgrades are not the contractor's responsibility to budget for. They come from your pocket, ideally from contingency or discretionary funds, not from money already committed elsewhere.
Costs After Completion
Decorating. Unless your contract explicitly includes painting and decorating, you'll be doing this yourselves or paying a decorator separately. For a full house after a major renovation, that can easily be £5,000-£15,000 including materials.
Landscaping and external works. The garden typically gets destroyed during a building project. Reinstating it, once the builders have gone, can range from a few hundred pounds to tens of thousands depending on what you want to do with it.
Snagging specialist. An independent snagging surveyor for a new build or major renovation typically costs £300-£600. It's money well spent, because they find things you wouldn't notice, and having that list before you release final retention gives you real leverage.
Furniture and window treatments. You've extended your kitchen. Now it needs furniture, lighting and perhaps new window coverings for the additional windows. These costs are entirely predictable but often forgotten when the build budget is being set.
New Service Connections
If your project involves a new connection to gas, electricity, water or drainage, the costs can be substantial. A new electricity connection from the grid to your property can cost several thousand pounds if underground ducting is required. A new gas connection in an area not previously served runs to similar figures. These are charged by utility companies, not your builder, and are entirely outside the build contract.
Always ask your contractor: "Is this included in your quote?" for every line item that could be ambiguous. Then get the answer in writing. "Yes" verbally and "not included" on the invoice is one of the most common sources of dispute at practical completion.