What do an architect, engineer, building control and party wall cost for an extension?

Fees are separate from the building work, and they are easy to leave out when comparing a contractor's quote with a budget. The table gives each fee line the cost engine prices for a single-storey rear extension and for a rear dormer loft conversion, both with a good-quality finish. The ranges are wide because the design fee follows the size of the job and the party wall line depends on the neighbours.

The totals at the foot are the lines added together, to the nearest five hundred pounds. The rear extension comes to £6,500 to £22,500 and the rear dormer to £6,500 to £21,500. Contingency is not in them: it is a separate allowance for what the building work turns up.

The design line is £4,000 to £10,500 for the single-storey rear extension and £4,000 to £11,000 for the rear dormer. It is priced as a share of the building cost, with a minimum, so it rises with the size of the job. It buys the measured survey, the design options, the planning or prior approval drawings, and the building regulations package that building control and the contractor build from.

Check what a fee stops at. Some stop when the drawings are done. Others go on to tender the job, visit the site and administer the contract, which costs more and is worth having on a complicated house.

"Architect" is a protected title. Under the Architects Act 1997 only someone on the register kept by the Architects Registration Board may call themselves an architect. Many good designers, technologists and design-and-build firms are not registered and may not use the title, which is why the engine's line reads "Architect or designer".

Registration gives you a regulated professional with a code of conduct, a complaints route and a duty to carry professional indemnity insurance. For a simple rear extension an experienced unregistered designer can do the job well. Whoever you pick, ask what they have drawn on similar houses, whether they carry indemnity insurance, and exactly what the fee includes.

The engineer's line is £1,000 to £2,500. It buys the calculations and drawings for the steel beams, foundations and any other structure, which building control needs before it will approve the work. Opening up the back of a house, or cutting a dormer into a roof, nearly always needs them.

An unusual structure, a long span or poor ground can push the fee up, because the engineer may need a site visit or a trial pit. The fee does not include the steel itself, which is in the building work.

Building control is £1,000 to £2,000 for a single-storey rear extension and £1,000 to £1,500 for a rear dormer. The work needs building regulations approval whether or not it needs planning permission. The fee covers checking the plans and a series of site inspections, from the foundations to the final sign-off, and ends with a completion certificate. Keep it: a buyer's solicitor will ask for it.

You can use the council or a registered private approver. The engine's line is based on council schedules, and private approvers can run higher, so get a fee in writing from each.

For the rear extension, the engine includes the council prior approval fee at £334, which is the statutory fee plus the Planning Portal's service charge of £85. For the rear dormer, it includes the lawful development certificate at £359, which gives written proof that the work was lawful.

A full householder planning application costs £548 where the work needs one, and the fee box below gives each council fee with the date it took effect. Whether your own job needs an application is a planning question, and the council is the place to check it.

The Party Wall etc. Act 1996 asks for notice in three common cases. The first is building on or astride the boundary, such as an extension up to a shared line. The second is digging foundations within 3 m of a neighbour's building and deeper than theirs. The third is a loft whose steel beams bear on the party wall, or which cut into it or raise it.

Each affected neighbour can consent or dissent. Consent in writing means no surveyors and a party wall line of nothing. A dissent, or silence for fourteen days, sets up a dispute: each side may appoint a surveyor, or both can agree one. You normally pay for both sides, which is why the engine's line runs from nothing to £0 to £6,000 for two neighbours.

A notice does not give a neighbour a veto. It protects them, and you, with a record of the wall's condition before the work starts. Serve notices early, because the Act sets minimum notice periods before work can begin.

The fees do not include the building work, which is priced separately, nor a contingency. They also leave out surveys beyond the engineer's visit: a drainage survey, a measured survey of an awkward site, a tree report or an ecology survey where the council asks for one. Costs for these depend on the site and come on top.

The ranges include VAT at 20% on professional fees. The council's statutory fees carry none. A quote from a firm that is not VAT-registered may be lower for that reason, and a quote that says "plus VAT" needs 20% added before you compare it.

How much does an architect cost for a house extension?

For a typical single-storey rear extension the engine's design line is £4,000 to £10,500, and £4,000 to £11,000 for a rear dormer loft. It follows the size of the job. "Architect" is a protected title under the Architects Act 1997, and a designer who is not registered can do the same drawings.

Do I need a structural engineer for an extension?

Almost always. Opening up the back of the house or building a dormer needs steel beams, and building control needs an engineer's calculations for them. The engine's line is £1,000 to £2,500, and the cost of the steel itself is in the building work.

Who pays for a party wall surveyor if my neighbour dissents?

Usually you do, for your surveyor and your neighbour's. The engine's party wall line runs from nothing, if both neighbours consent in writing, up to £0 to £6,000 for two neighbours. Each side may appoint its own surveyor, or the two can agree on one to keep the cost down.