House Extension Costs

House Extension Costs Table

There are many factors to consider when pricing a house extension as each situation is unique, but our table below with give you a quick idea of what you can realistically expect to pay.

Keep in mind that the following is an estimate.

You can contact us for a fast, free, and no-obligation quote.

General Cost of Extensions Per Square Metre

Extensive experience shows costs range from £1000-1800/sqm.

Site survey: from £350
Planning: from £850
Construction from £50K
Total costs: @£1000-1800m²

Cost of Side
& Rear Extension

The cost of side and rear extensions usually average around £1700/sqm.

Site survey: from £400
Planning: from £1100
Construction from £70K
Total Costs about £1700m²

Loft Conversion Cost

Loft conversion costs vary greatly, but usually average £1000/sqm.

Site survey: from £400
Planning: from £950
Construction from £40K
Total Costs about £1000m²

Cost of Single Storey Extensions

The cost of a single storey extension averages to £1100/sqm

Site survey: from £400
Planning: from £1200
Construction from £65k
Total Costs about £1100m²

Cost of Multi-Storey Extension

Multi-storey extensions require more structural work therefore cost on average £1800/sqm.

Site survey: from £500
Planning: Varies
Construction from £75k
Total Costs about £1800m²
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Things to keep in mind when costing

How much do architects charge for extension plans?

Architects may offer to submit your planning application to the council – and would pass the standard council charge to you – but should not add any VAT. They may liaise with the planning officers for no charge – or at an hourly rate – depending on the architect. This saves you the job of submitting the planning application yourself.

All-in-one Service

If the architect doing the planning drawings for your home extension ideas also offers pre-build and construction support services like building regulations drawings, tender packs and contract administration to manage your build, that will give you the benefit of a one-stop shop, and should save you money in the long-run over your cost of extension planning and design, rather than hiring and coordinating with people for different tasks.

Click here for more information on the role of Building Control with your build.

What about house extension construction costs?

Apart from architect fees, people often want a ball-park figure per m2, for building estimates. £1000-£1500/m2 is a fair estimate, but a lot depends on the type of construction – for example, the cost per metre of a stud wall would differ from that of a new bathroom.

If you have been asking yourself: “How much does an extension cost to build?”. we can give you an idea. Builders sometimes quote £35-50K for the shell of a single-storey rear extension, and the quote for the first and second fix would depend on your choice of materials. It is possible you may get cheaper quotes, but the builder may not include the cost of everything, or they might underquote the cost of labour.

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What are standard architect fees for an extension in London?

There are architects whose portfolios are wholly in exclusive and contemporary areas, they may source sustainable, well-tested materials, or use smart home technology. This class of architects can charge fees that reflect the above; for example, one practice charges £1500+VAT for 2 hours of their time to do a site visit and a Feasibility Study.

Another practice charges as much as £4999 for a bound Feasibility report, and that does not even include the architectural drawing! That practice offsets the high fee by offering a site consultation for only £125.

Still another practice charges as much as £640+VAT for a site consultation, and keep in mind that that is not even the technical measured survey.

Keep your feet on the ground by looking for more down-to-earth services. Such architects may do a mini-feasibility study online for free for you and would charge less for more standard projects, but it is still an outlay to be considered. In those instances, you would be advised to check things like their record for securing approvals, RIBA or ARB status and client testimonials. Recommendations from friends are usually a good starting point.

Some architects do a site consultation for free, but beware if this is absorbed into another cost to recover their travel and time expenses. Others will charge for a site consultation in the region of £250+VAT, but may refund that fee on instruction. For these practices, the architect hourly rate could be around £120+VAT. Most architects would charge for the site technician’s survey, because it takes a good couple of hours to document the whole site properly with laser equipment and strategic camera shots. This could be from about £350+VAT, although some may charge less.


The above-mentioned service costs are the usual charges you can expect from UK architects for extensions. There may be some additional costs as well that may be mostly hidden. If the firm is asked to administer your project or manage project, they can charge you an additional 10% to 15% of the contract amount in the form of fee. You should expect to pay almost equivalent even when you choose to pay by the hour. All this can mean that the overall costs can be up to £5000 even before work on the extension begins.