Building a home extension can increase the value of your property and offer a more cost-effective alternative to moving house.

As well as the financial benefits, an extension will significantly improve your existing living space and make your home a more enjoyable place to live. It could be designed to create an open-plan kitchen-dining-living space with better access to your garden, or an extra bedroom, home office or children’s play area.There are so many potential options.

As a general guide, a mid-range single storey extension will cost around £2,000 per square metre, excluding interior finishes and fittings, fees and VAT in most parts of the UK, increasing to around £2,500 per square metre in London. A budget-range extension will cost approximately 25% cheaper while a high-end extension can be double the cost. Expect a two-storey extension to be about 50% more expensive than a single storey extension. A simple basement extension will start in the region of £3,500 per square metre.

Continue reading for more information -

‍

Home extension costs

What affects home extension build costs?

How to reduce costs without losing design quality

What’s involved in building a home extension?

How do I find and appoint a builder?

Key considerations before building a home extension

FAQs

‍

‍Household Extensions - Approximate Build Costs (£/sqm)

‍

Firstly, it’s important to note that these costs (per square metre) are for the building shell only. Full project costs are covered in the sections below.

A single-storey extension in most parts of the UK will start at around £1,400-£1,600 per sqm, excluding kitchens and bathrooms. In London, prices start at around £1,700-£2,200 per sqm.

The cost per square metre of a two-storey extension is similar to a single-storey extension: £1,500-£1,700 in most parts of the UK, and £1,800-£2,300 in London.

It can be difficult to gain planning approval for a two-storey extension, but there are economies compared to a single-storey extension with a similar floor area. For example, foundation, below-ground drainage and floor construction costs can be minimised as the building footprint can be more compact.

Basement extensions will cost in excess of £4,000 per sqm in London and £3,500 per sqm in the rest of the UK. Due to technical complexities and design risks, basement extensions are very expensive to build and are only a realistic option in high-value locations where works costs can be offset by property values.

Side return extensions are covered in detail in our article - A guide to side return extensions

‍

Kitchens and bathrooms

Add about £10,000 to the above square metre costs for a medium sized, low- to mid-range kitchen, inclusive of appliances. High-end kitchens designed and installed by a named manufacturer will cost more than £30,000.

A simple ground floor W.C. can be installed for around £3,000, subject to below-ground drainage connections.

A shower room or compact bathroom with mid-range fittings and finishes will cost about £5,000-£7,000. A similarly fitted-out large family bathroom will cost in the region of £10,000.

‍

Interior finishes and fittings

The above low- and medium-end square metre costs cover simple interior fittings, decorations and electrics.

Higher-end finishes, such as natural stone tiling and fitted joinery, will increase costs. As will more sophisticated lighting and audio-visual installations.

‍

Wider refurbishment costs

It’s important to note that the above costs are for the extension parts only. In many cases, the extension will form part of a wider refurbishment and the cost of these works should be added to your total estimate. As a very rough guide, budget in the region of £900-£1300 per sqm for the renovation of existing areas, but it very much depends on the extent of the works. If you’re taking out loadbearing walls and installing steelwork, for example, the cost can be much higher.

‍

Landscaping works

A cost that is frequently underestimated is landscaping works. Add about 20% of the total extension build costs to cover extensive landscaping work. If you’re lucky enough to have a well-stocked, mature garden, you could reduce this to around 10%.

‍

Professional fees and other costs

The cost of all professional fees will be around 15% on top of construction costs: approximately 10% to cover architect’s fees and 5% for other consultants (structural engineer, quantity surveyor and party wall surveyor). You could save on architect’s fees if you, or others, decide to manage the works yourself - read on to find out more.

Administration costs for statutory permissions (planning and building regulation approvals) equate to a further 1%.

If you are building close to a boundary, you may require the services of a party wall surveyor. Budget a further 3% to cover their fees.

Home insurance policies do not usually cover building works. Allocate about 1% of your budget for non-negligence cover which can help protect you against most eventualities while the builders are on site.

‍

Value added tax

Building an extension will normally incur VAT at 20%. If you are extending a property that has been vacant for more than two years, or the works involve the conversion of an existing dwelling that is being subdivided into units, you may be eligible for tax relief.

You can only claim relief by using a VAT-registered builder.

‍

A summary of home extension costs

Project costs for a home extension, inclusive of full architect’s services (concept design through to site completion) can be roughly broken down as follows:

• Build costs – 60%
• Professional fees – 15%
• Statutory approvals – 1%
• Party wall – 3%
• Insurance– 1%
• VAT– 20%

‍

What affects home extension build costs? 

• ‍Location. As noted above, building in London and the South East is likely to cost more than other parts of the UK. Extensions built in a designated area, such as a conversation area or an area of outstanding natural beauty, will require a higher standard of design and construction. Build costs and professional fees will therefore be higher.‍
• Size and shape. A small box-shaped extension will be more economical to design and build than a larger, more complex design. ‍
• Site constraints. Ground conditions, typography, trees and vegetation, structural work, below-ground drainage, utilities and site access can all influence costs.‍
• Specification. Your choice of external cladding materials and glazing and internal fixtures and fittings will make up a significant proportion of your overall budget. ‍
• Method of construction. Traditional construction - brick and block cavity walls, suspended timber floors, pitched roofs etc - is the most economical building method in the UK. Modern methods of construction, such as engineered timber frame construction and SIPs, is more expensive but savings can be made on labour costs as structures can be erected and insulated very quickly.‍
• Market fluctuations. The cost of building materials is subject to global market forces and prices can fluctuate from one month to the next. The same is true of the labour market. During times of skill shortages, the cost of skilled labour, in particular, can dramatically increase.‍
• Build route. Here you will need to balance quality and time considerations as well as cost. Traditional lump sum (fixed price) contracts, where a main builder is appointed to deliver the entirety of the site works and the architect is retained to project manage, offer a high degree of cost certainty and build quality.

Savings can be made by asking the builder to project manage the works themselves, or you could oversee the building work yourself. Further savings can be made if you directly appoint the individual trades yourself, but this can be risky. Both options can result in a loss of quality.

Where at least 30% of the works are DIY and the rest are done by directly appointed labour, self-building can achieve big savings. The main consideration here is being realistic about whether you have the time and ability to complete the works. Appointing a builder to finish off incomplete building work can work out more expensive than a traditional lump sum contract.

How to reduce build costs without losing design quality

1. Maximise the quality of your existing space as much as possible and only extend to the size you need. 
2. If your budget is tight, keep the general design as simple as possible so you can concentrate funds into quality materials, fixtures and fittings - things that will make a real difference to your day-to-day life. A complex design will eat into your budget fast, leaving little left over for the things that matter.
3. Keep glazing to a minimum. Glass is expensive and less thermally efficient than solid construction. The best extensions only use it where it is necessary – to frame a view, to introduce light to a dark area, or to provide access to a specific outside area.
4. Minimise groundworks as much as possible. If you require a lot of additional floor area, consider building over two storeys instead of one. It will keep the building footprint nice and compact which can save money spent on below-ground works.
5. Simplify connections to existing services – drainage, plumbing, electrics, utilities – and retain existing service runs where possible. Position new kitchens and bathrooms next to existing plumbing connections and stack kitchens, W.Cs and bathrooms above one another (if building a two-storey extension) to minimise on new pipework and below-ground drainage connections.
6. Minimise wet trades. These, such as plastering, need to be carried out by skilled tradespeople and take time to install. Building the external walls of your extension in timber frame construction instead of traditional brick and block, and lining some of the interior surfaces with timber panelling, such as plywood, (a dry trade) can speed up construction and save on labour costs.
7. Source local materials, products and labour wherever possible. This is good for sustainability and will reduce supply risks.
8. Dress-up off-the-shelf kitchens and built-in storage. Reduce costs by installing off-the-shelf kitchen units, drawers and storage components, and finish with bespoke door fronts, stone worktops and quality fittings. Similarly, for built-in storage, set out new walls and ceilings to accommodate an off-the-shelf system and install bespoke door fronts to save on joinery costs.
9. And finally, appoint a skilled architect. A well-designed and well-managed project will not only save you money, but also time and stress.

What’s involved in building a home extension?

Below is the typical order of events for a home extension project. Items may vary depending on your project specifics.

1. Write a brief setting out your requirements.
2. Appoint an architect.
3. Appoint a surveyor to carry out a measured survey of the property.
4. Develop design options in accordance with your brief.
5. Agree a preferred design and seek initial cost advice.
6. Obtain planning permission, if applicable.
7. Appoint a structural engineer and any other consultants, as required.
8. Prepare technical drawings and specifications.
9. Prepare a schedule of works (an itemised schedule for builder pricing).
10. Agree party wall awards, if applicable.
11. Obtain building regulations approval.
12. Issue technical information for tender.
13. Review tenders and select preferred builder.
14. Refine drawings and specification and agree final contract price.
15. Prepare and sign building contract.
16. Arrange insurance to cover building works.
17. Start on site.

How do I find and appoint a builder?

Finding a good-quality builder for domestic works is not easy. The nature of the work attracts small companies who can vary greatly in ability and approach.

Architects prefer to deal with professional firms who produce good-quality work and are therefore a great source of contacts. Most architects will be able to suggest at least three suitable building firms to tender for your works.

In times when builders are in high demand, it can be very difficult to organise a competitive tender. Good builders can pick and choose their projects and they will simply not price if they are required to compete with other firms – the time expense is not worth their while in a buoyant market.

In such a situation, it’s often better to select a builder that is the right fit for you and your project and agree a price with them direct. Your architect and/or quantity surveyor will be able to analyse prices to make sure estimates are reasonable.

When appointing a builder, it is always advisable to use an industry standard building contract. A building contract not only provides legal protection if something goes wrong, but also helps to structure the build phase of a project. It sets out:

• The scope of the work to be undertaken.
• The cost of the works and when stage payments are to be made.
• The roles and responsibilities of those involved.

The type of contract used should be proportionate to the value and complexity of the proposed works. JCT Minor Works is suitable for many extension and refurbishment projects. An architect will be able to advise on what is appropriate for your project.

Key considerations before building a home extension

• What is the purpose of the extension?
• Can I maximise my existing floor space without building an extension?
• Can I achieve what I want on my budget?
• Will the extension add value to my home?
• How much space do I need to add to fulfil my requirements?
• How will the extension affect my existing floor space?
• Will the extension block light into existing adjoining rooms?
• How will the extension affect my neighbour’s properties?
• Is it cheaper to move to a larger property or to extend my current property?
• Where do I need the extra space - at ground floor level, above, or both?
• Do I need planning permission?
• Do I need party wall agreements with my neighbours?
• How long will the building works take?
• Do I need to move out while the works take place?

‍

FAQs

Do I need planning permission to build my extension?

If you live in a house (not a flat) which is not listed or located in a designated area (i.e. conservation area) then there’s a good chance you will not require planning permission so long as the design of the extension meets permitted development guidelines.

‍

Do I need building regulations approval to build my extension?

Yes. Building a new extension will require building regulations approval.

‍

Do I need to appoint a structural engineer?

Yes. A structural design and calculations are required for building regulations approval.

Do I need to appoint a quantity surveyor?

For projects over £100K, it is advisable to appoint a quantity surveyor to advise on costs prior to planning submission, and then to assist with price analysis following completion of the technical design and the return of builder tenders.

Will an extension add value to my home?

Yes. An additional bedroom or larger kitchen-dining or living space will undoubtably add value to your home. The key question is, will the increased value exceed the cost of the building works? In many cases the answer is also yes. Historic property price inflation and high moving costs in many areas of the UK means it’s usually more cost effective to improve your existing home than move to a larger one in a similar area.

‍

Do I need an architect to manage the work on site?

This is down to personal choice. If your goal is to simply add more floor space, no. If you want to achieve the best possible finished result, yes.‍

‍

Will my extension require party wall agreements with my neighbours?

If you are extending close to an adjoining wall or boundary, yes, a party wall agreement is likely to be needed.

‍

Please see links below to other articles which may be of interest.

Do I need an architect?

How to write an architectural brief - a homeowners guide

How big can a home extension be without planning permission?

Credit - Main image - Photo by Pedro Miranda on Unsplash

Note: Cost figures included in this article are for general guidance only, and were estimated from personal experience and online research. Every project is unique.

‍

About the author: Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.

The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled residential architect to provide specific expert advice for your project.

‍