How a House Extension Budget Is Actually Built

Budget one: getting to a design that can be built

Measured survey, architectural drawings, structural engineer's calculations, and where required a planning application fee and drainage approvals. On a listed or conservation area property, add heritage input.

This budget is spent before a single brick is moved, and skimping on it is the most reliable way to overspend later, because vague drawings produce vague quotes and expensive variations.

Budget two: consents and neighbours

Building control charges apply to every extension. Party wall matters apply wherever you build up to or across a boundary shared with a neighbour, and if the neighbour dissents and appoints their own surveyor, you generally pay for both.

Serving notices early is free. Serving them late costs programme, which costs money.

Budget three: the shell

This is what most people mean by the extension: groundworks and foundations, drainage, masonry, roof, structural openings into the existing house, windows and doors, and making watertight.

It is the part where the site has the most influence and the homeowner has the least. Once the design is set, the shell price is largely a function of ground conditions, access and structure.

Budget four: services and fit out

Heating, electrics, plumbing, plastering, flooring, joinery, decoration, and any kitchen or bathroom within the new space. This is the budget most under estimated at the start and the one with the widest range, because it is almost entirely specification led.

Underfloor heating and the controls to run it properly · Lighting design, which is a specification not a default · Flooring, where the same area can differ several fold in cost · Fitted joinery, which is usually priced per run rather than per project

Budget five: contingency, and why it is not optional

Existing buildings hide things. Old foundations that are shallower than expected, a drain not on any plan, rot behind render, or a wall that turns out to be doing more work than anyone thought.

Holding a contingency is not pessimism, it is the difference between a discovery being an inconvenience and being a crisis. A project that finishes without spending it has simply come in under budget, which is a good outcome.

Where the money is best spent

If the budget will not stretch to everything, protect the things that are hard to change later: the structural opening, the glazing, the insulation and the services layout. Kitchens, flooring and decoration can be upgraded in five years. A wall in the wrong place or a mean opening cannot.

Frequently asked questions

How much contingency should I hold?

Enough that an unexpected foundation depth or an unmapped drain does not stop the job. On an older property, hold more, because there is more that nobody can see until the ground is open.

Are professional fees really necessary?

Drawings and structural calculations are required for building control approval, so they are not discretionary. Their real value is that they let several builders price exactly the same thing.

What is not usually in a builder's quote?

Commonly professional fees, planning and building control charges, party wall surveyor fees, and sometimes kitchens, appliances, flooring and decoration. Check each one explicitly rather than assuming.

Should I fix the price or work on a cost basis?

A fixed price against a clear specification suits most domestic extensions, because it puts the pricing risk with the party best placed to manage it. It only works if the specification is genuinely clear.