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Cost of Building a Single-storey Extension

Building an extension is an excellent way for families to get more space from their homes without moving. Adding an extension to your property can improve your home as you will add space and light to the building.

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Introduction to Building a Single-storey Extension

You create more living space, perhaps giving current occupants more room, or accommodating a growing family or elderly relative.

It is often more economical than moving house, as buying a new property usually incurs stamp duty and estate agent fees, which can be a considerable expense. So extending your home makes perfect sense both financially and aesthetically.

Whatever the reason, you will find that the price of an extension can vary enormously. A huge factor in this is location; it's all about location, location, location.

You can install a single-storey rear extension to your property, (single storey kitchen extensions are always popular), to the side, as a single storey side return, or, even to the front of your property, depending on the planning regulations for your area.

How Much Does It Cost to Build a Single-Storey Extension?

There are many things to consider with single-storey extension prices, and costs may be higher than anticipated. Different aspects will affect the price, such as the single-storey extension roof types available to marry in with your current house's particular style.

You may need a particular window style to be replicated to blend seamlessly with your house's present style. Small details matter, even down to the door hinges and facing stone you choose, all of which will have cost implications.

You will inevitably have to employ the services of an architect; again, this is costly. However, if the extension's overall design is not considered, styles can jar, harming your property's look and feel. Builders generally like to be guided by an architect's plans.

To arrive at the building's true average cost, you need to calculate the cost of your single-storey extension per m2. An architect will provide these figures for you and assist you throughout the build should problems arise.

Some justifiable single-storey extension costs, whilst high will eventually provide savings from the resulting benefits, such as low maintenance costs, more room, higher resale prices and extra curb appeal.

This article may be helpful if considering a single-storey extension, but bear in mind costs vary greatly, depending on location.

Some basic online research will reveal the best single-storey extension ideas, as well as builders offering their services, with the capabilities you require. You will need to get several quotes to find the right company for you. You could save up to 40% by comparing quotes. This article is here to offer advice and help.

We will now break down the costs involved in building a side extension.

Extension costs will vary, depending on what you want. Smaller or less complex extensions work out cheaper. Where you live also plays an essential factor. If you live anywhere near London, your extension will cost considerably more. But there are guidelines or rules to help. Below is a rough guide for each type of extension to help give you a general idea.

For most of the UK, a current cost estimate for a single-storey extension can be between £1,350 and £1,950 per m2 However, if you live In London and the South East, this could escalate to anywhere between £1,800 and £2,300+ per m2. A high-end extension will cost you considerably more.

Kitchen or Bathroom Extension?

If you want your single-storey extension to be used as a kitchen or bathroom, plans and permissions such as wiring, plumbing and other installation costs need to be considered. These things need to be done professionally by qualified contractors.

For a bathroom, you'll need to add a minimum of £5,000, depending on your required finishes and what you require.

Kitchens usually cost more than bathrooms, with prices starting at around £10,000 for a low to the mid-range kitchen. Both kitchen and bathroom extensions will vary according to your tastes.

Use a house extension calculator – available online - to work out the price of your ideal extension. On average, an excellent finish is 40% more than a standard finish.

Windows can also be an expensive yet often overlooked addition to an otherwise modest extension cost. Ultimately your extension costs will depend on factors such as:

  • size of the extension.
  • quality of the build: standard, good, excellent.
  • The amount of glazing you'd like.
  • Internal fixtures and fittings.
Extension Size Area Cost
Small 3 x 5 m £16k - £21k
Medium 4 x 6 m £26k - £34k
Large 6 x 8 m £52k - £67k

The price of an extension is between £1,100 to £1,400 per square metre. So, a 20msq extension (4m x 5m), will range from £22,000 to £28,000. The prices don't include VAT, usually charged at 20%. The price is affected by the materials used, the type of design, the quality of finish and your location.

These are the main factors that will affect the cost of building an extension and give you an idea of cost per m2 so you can estimate your potential costs. To help you get an idea of how much your extension will cost, here are five top tips that will help you set a budget before you begin.

Depending on your location, a simple extension should cost around £1,000–£2,000/m2. The standard of specification you choose will influence the build cost, as laid out in the table below:

Quality of Build Price per m2
Basic Specification £1,320 to £1620/m2
Good Specification £1,700 to £2,000/m2
Top Specification £1,800 to £2,500/m2

Types of Single-Storey Extension

If you need space for a new kitchen or luxury bathroom, a well-planned extension can transform even the smallest home.

Here are a few thing to consider regarding planning permisson:

  1. The area of side and rear extensions must be less than 50% of the total free area around the house.
  2. The exterior appearance must blend in with the existing house and the neighbourhood.
  3. The width of side extensions must be less than 50% of the width of the existing house.
  4. The max length of rear extensions is 3m for a terraced or semi-detached house, up to 4m for detached homes.
  5. The height of single-storey extensions must be less than 4m. If the distance to the boundary is less than 2m, only single-storey extensions are allowed under permitted development.
  6. Balconies and terraces are never allowed within permitted development rights.
  7. Loft and basement conversions have specific permitted development requirements. You might consider seeking an expert's advice if you are planning a loft or basement conversion.

Rear Extension

It's a good option for detached and semi-detached houses. Terraced house extensions at the rear of a property are sometimes feasible, too.

Wrap Around Extension

A combination of the rear house extension and side extension. It's feasible for semi-detached or detached house extensions.

Side Return Extension

Terraced and semi-detached properties often have redundant space to the sides, which can be incorporated into your building. A side extension uses this redundant space at the side of a house to extend without using any garden space. It's an excellent option for a ground floor extension, to implement a change of character on a ground floor without compromising the back garden area.

The process will usually entail knocking walls down to create a larger, potentially open-plan zone. As the main pricing calculation for an extension is based on floor-space, where you site your extension will not significantly impact the build's overall cost. Factors that affecting pricing depends on the design of the extension.

Full Width Extension

In some cases, it's possible to extend across the whole breadth of the house – allowing you to maximise the amount of living area you can add to your home.

This project will typically require you to arrange a party wall agreement that protects the neighbouring properties.

Creating an L-shape Extension

Many projects involve filling a courtyard area with extra living space, but if you've got a well-sized garden, then extending out with a long, thin zone could help you add more room whilst also maximising access to the garden.

Glazed or Garden Room Extension

If your key goal is to add an airy, light-filled space, plenty of glazing is likely to be a key goal. With careful planning, the glass box tends to suit both contemporary and traditional homes.

Alternatively, you could go for a conservatory or orangery for the heritage feel. Garden room style extensions are usually very design-focussed, typically single storey with large windows or bi-fold doors to blend the inside with the garden.

However, several factors will impact on the final price of a garden room project. Consider your specifications carefully to prevent exceeding your budget.

Extension over an existing storey (i.e. over a garage)

If you have a garage, you can easily extend over it at a fraction of the cost of starting from scratch.

You will need to check that there are foundations that can cope with any extra load, but often your garage will already be on good foundations and there is only need to build upwards to get the single-storey extension benefits.

Linked Building

Your new space doesn't have to be integrated into the existing house. In fact, in some circumstances, it may be better to create a separate building that links back to the original structure, perhaps via a glazed corridor. This kind of approach can work exceptionally well for heritage homes, where the planners may be keen on the IDF, creating a distinct new zone that's subservient to the main house.

Many tradespeople claim that a second storey will be priced at only half or two-thirds of the first-floor cost. Tradespeople tend to disagree, saying that builders price jobs based on the total square meterage. A two-storey extension with 20msq of floor space should cost £44,000.

Although the ground floor is more labour intensive because of the groundwork and foundations that must be completed, several additional costs are involved in a second storey that does not apply to a single-storey project. Hiring scaffolding, working at height, structural integration with the original building, changing the roof, are all factors that serve to increase costs.

Traditional "brick and block" construction has been the most common building in the UK for nearly a century. But timber frame projects are becoming more popular. The masonry construction method uses a basic structure built from blocks. Then it is clad with brick or other exterior cladding added around it. On the other hand, Timber frame uses wood to construct core panels assembled on site.

Many variables will affect how much your extension will cost, such as:

  • Soil type which can affect foundations.
  • What is the extension's use and fit-out cost?
  • How much glazing do you need?
  • Size (it's generally cheaper per unit the bigger the area.
  • Location (the most expensive areas can be 30% more than more affordable places for labour).

Who you have managing your extension project will affect how much your extension will cost.

It is possible to save around 40% if you build yourself. However, get an estimate or a quotation for any contractor fees you think you will need to call in. Plumbing and electrical wiring of your property are usually best to let to the professionals.

A quotation, on the other hand, is an actual price. Quotes will itemise the work to be done, provide a breakdown of costs and a total, and state whether VAT is included. Don't Forget to Add VAT to the cost of your extension. VAT on labour and materials will be charged at the standard rate of 20%.

If you occasionally use local tradespeople and they are not VAT registered, you can save the 20% VAT on their labour, but will still pay VAT on materials. Some projects are eligible for VAT relief, such as: - (BLANK)

To benefit from VAT relief f, you must use a VAT registered builder — you can't reclaim the VAT yourself.

What Does Building a Single-Storey Extension Involve?

Builders will not usually separate costs for elements such as groundworks, which are rarely done separately from the rest of the project. If you have an extension built, most will pay their tradesmen in stages, typically after some of the following elements are competed. This is not an issue if you are doing the work yourself.

Design & Planning Permission Costs

If an architect or architectural technician draw up plans for you to follow, this may cost you around £500 to £1,000 depending on the scale and complexity.

Planning permission isn't always necessary for an extension. Permission from your local authority can cost around £200 depending on your location, such as living in a conservation area.

Building regulations are the essential standards to which all UK construction projects must adhere. Building Regulations ensure buildings are safe, structurally sound, and water and energy-efficient. The cost will vary depending on the build's size, but will typically be around £300 to £500.


Preparing the site and digging foundations is essential to ensure safety and with structural integrity. Costs for this phase can be difficult to separate, if there are complications at this stage, such as moving drains or dealing with tree roots, it can slow down the process, thus increasing building costs.

The Shell

The basic structure is built using either brick and block or a timber frame. The roof is added, and interior elements such as plasterboard and a subfloor ensure a watertight structure.

The main building material choice doesn't affect much of the cost for an average extension project.


Fitting out features like skirting boards, doors, electrical sockets and switches, and radiators add to the cost. All will depend on whether you want to purchase high-end materials or not. Sliding or bi-fold doors can cost thousands of pounds per metre, while simple French doors will cost much less.


Costs for finishing off the extension are limitless; every individual element has a different price. Costs will increase if you add a new kitchen or bathroom to the extension. A new kitchen will cost between £7,000 and £25,000, and a bathroom will cost between £3,000 and £10,000.

Be fully aware of the job scale and the costs involved if you decide to do it yourself. Set yourself a budget and get several quotations for the work that you will require help with. Have a contingency set aside to help you cover any extra expenses that could, and inevitably will, arise through delays or other issues.

DIY is a major way to save money on a project like an extension, but you will need to put in more time and effort to save money. You could choose to do some of the jobs yourself, such as the decorating. You could source materials, such as tiles and flooring yourself. But it is always worth remembering that a tradesperson may save you money using trade discounts.

Additional Considerations

  • Costs associated with finishing off the Single-storey extension.
  • A single-storey extension will have to be sealed or coated to protect it from the weather. There could be extra costs involved for these additional steps.
  • Ensure that you are satisfied with the work, and it's finished to a professional standard before signing off on your project.

DIY Single-storey Extension Build

Before you start building an extension, it is essential to collect all the preliminary information you can find. DIY is a way to make significant savings on your self build and can save you many thousands of pounds, as long as you know what you're doing.

You may need to combine physically demanding tasks of groundworks, bricklaying, and roofing with skilled plumbing and plastering tasks. You can mix your own labour with professional labour where required by substituting labour costs with DIY. Employ help with the rest of the building work when necessary.

Whether you are building, an extension or making structural alterations to your home, you'll need Building Regulations approval from your local building inspector. Planning approval means getting permission to build an extension or new dwelling from your local authority. Building Regulations approval ensures that the proposed structure is built according to plans and regulations.

An essential part of getting approval is benefiting a building inspector to carry out regular site visits. Building inspectors have a complete working knowledge of the many regulations involved in building a house or extension. Their job is to ensure that the completed project has been built in full compliance with each of them.

Some crucial factors to be considered before starting your project. Here is a step-by-step guide to extension construction from start to finish.

  • Access: You may have problems with planning permission if your building extension limits off-road parking.
  • Getting materials to the plot - can you or the builders easily access the site, or will all materials need to come through the property?
  1. Your extension cost depends on many things, not least the complexity of the design. Roughly speaking, a cost of £1,000–£2,000 per square metre is usual. You may want to employ an architect/designer or structural engineer, which will incur additional costs. Balance the budget for your extension against the potential increased value to the property.
  2. For a small extension, use an interest-free credit card as finance. For larger extensions, try a personal loan, which is generally available up to the value of £25,000. You could re-mortgage your home or consider taking out a secured loan to get the required capital if you need more money.
  3. Your building can be treated as a conversion if your property has been empty for two years, reducing VAT to 5%. However, to do this, you or your builder will need to be VAT-registered.
  4. Not all extensions require planning permission. Permitted Development Rights mean you may be able to begin building without telling your local authority. Check Gov. sites for more information. However, extensions built on National Parks or Conservation Areas properties will usually be subject to reduced Permitted Development Rights.
  5. Your local planning office will advise you whether you will require planning authorisation. Changes to a listed building will almost always require planning permission.
  6. If any structural defects or faulty quality become apparent, you will only have recourse if a warranty was provided before work commenced.
  7. For smaller extensions, an Architectural technologist might be suitable - and are usually cheaper than a fully-fledged architect.
  8. Choosing home designers that are affiliated with a reputable architectural body is a smart idea. To design your own extension yourself, you will need your own PII.
  9. You will probably need an expert builder's services during your build for any tricky aspects you are not confident enough to take on yourself. Avoid day work rates as they can push costs up. If VAT is added to the bill, make sure the VAT registration number is detailed on the receipt.
  10. It will be beneficial to find a group of tradesmen with a range of skills. Having access to people who are also skilled electricians will eliminate delays caused by non-availability of electricians. Also, remember you are responsible for keeping the site tidy and taking out applicable insurance.
  11. Because of the disruption building causes, you may decide to move out for the duration. Remember to cost this in when drawing up your budget.
  12. Work out if your existing boiler can cope with the additional demand of the extension. Also, decide if you need any additional electrical circuits –usually only necessary when adding a kitchen.
  13. Inform your insurer if you plan to extend your property. Building insurance won't cover events occurring during major building works.

Potential Problems and Pitfalls

  • On-site mistakes can happen.
  • Factor in your lost earnings during the build.
  • Your progress will be slower than using professionals.
  • Your quality of build might not be to a professional standard.
  • Warranty and building inspectors are likely to be more stringent in their checks.
  • Living on-site could slow down progress.
  • Ensure you hire the right designer for your extension: it can make a huge difference in the final results.

Updating the Access

Access is crucial when building works start. Workers and materials need access to enter the construction site. Also take into account disruption to the neighbouring properties and public areas, as any disputes will waste valuable time and can hold up your building indefinitely.

Get professional advice if you think your home extension has access issues before, during and after construction.

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Sam J

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