Cost of building an extension

If your property could benefit from some extra space, building an extension may be the best way to do it.

The possibilities here are endless, and your extension can be made to suit your budget, location, and individual requirements. For those looking to extend their kitchens or ground floor living spaces, a single storey extension may be the solution. And for households outgrowing their bedrooms or bathrooms, an extension built on top of an existing garage or a double storey extension constructed from the ground up may be the answer.

The cost of an extension will vary depending on several factors, namely:

  • How extensive the work will be,
  • Where you live, and,
  • Who completes the work.

Using the Tradesmen Prices form at the top of this page, you can access 3-4 extension quotes from trusted tradesmen and contractors experienced in building extensions who will be able to get the work done on time, and to a high standard.

Types of Extension

There are several different types of extension and they’re generally categorised by either how much space they offer or where on the property they are situated. Bear in mind that extensions can be built to your individual needs and you may be able to combine elements of one with another to achieve your desired effect.

  • Single Storey Side or Rear Extension
    This type of extension takes up a single storey and is located either at the rear or to the side of the property.This type of extension is the most popular in the UK and is usually chosen by those who can afford to lose a few metres of their garden for the sake of extending floorspace.Roof options include either pitched or flat, though this may be affected by planning permissions, as extensions must be built in keeping with the aesthetic value of the rest of your property.
  • Double or Multi-Storey House Extension
    This type of extension offers the opportunity to open up rooms on both floors for maximised floorspace.Planning regulations are more stringent when it comes to double or multi-storey extensions, so careful consultation with neighbours ahead of the design itself is invaluable in ensuring the build can go ahead. In the UK, a double or multi-storey extension must not impede on light into a neighbour’s garden or property.Similarly, extensions which could block the view of neighbouring properties may be denied planning permission, so make sure that your architect is aware of the build limitations before they start to collaborate with you on a design.
  • Garage Conversions or Extensions
    For those with an existing garage built with a solid structure, the option of converting and/or extending the garage itself can be an attractive one.Typically, converting a garage will cost between £10,000 and £15,000 less than building a new, ground-floor extension making it a cost-effective choice for those with tighter budgets.However, it’s wise to check whether sacrificing car storage space for floorspace is worth it – though conversions undoubtedly add value to a property, you may find that where your house is located, having off-street parking will actually be of greater benefit than being able to offer an extra room.

Cost of an Extension

Extension costs vary, though it’s rare that you’ll find the price falls below £30,000. Because an extension requires a lot of work to be completed – often by a number of different contractors or tradespeople – the cost of labour can sometimes exceed that of the costs of building materials.

Below, we have listed the approximate prices you may want to consider when planning the building of a single-storey extension in a table. For estimating the cost of a double or multi-storey extension, you may wish to double the relevant costs (such as for plastering and windows.)

Work Cost
Kitchen (depending on the specification) Between £5,000 and £20,000
Bathroom (according to the fittings chosen) Between £4,500 and £11,000
Shower Room (again, depending on fittings chosen) Between £4,500 and £11,000
Flooring (these vary greatly) Usually around £25 to £100 per square metre
Walls and Ceilings (for plaster, dry-lining and paint) Around £85 per square metre
Bi-Fold/Sliding Doors Between £1,500 and £2,000 per linear metre
Heating (if you need a new boiler) Around £2,300

When budgeting for an extension, you will also need to consider the possibility of professional project fees. This is especially important if you are looking to build a double or multi-storey extension as these must be designed with stringent building regulations in mind.

For an amateur, this leaves far too much room for mistakes to be made which is why employing a professional architect and structural engineer can be invaluable to the success of your project. The costs for these can be estimated as follows:

Work Cost
Planner Drawings £2,700 – £4,000
Construction Drawings £2,700 –  £4,000
Structural Engineer £500 – £1,000
Measured Survey £500 – £1,000

Typically, professional project design fees make up around 3%-7% of the overall construction cost.

Additionally, you may need to budget for a number of planning fees, depending on your property status, location, and the regulations imposed by local authorities. These may be as follows:

Work Cost
Planning Fee for Residential Extension £206
Certificate of Lawful Development £103
Fee for Discharging Planning Conditions £28 per request
Tree Report Upwards of £800
Flood Risk Assessment (Within Flood Zones) Upwards of £800
Ecology Report (if required by Local Authority) £800+
Archaeological Report (if in area of Archaeological Interest) Up to several thousand pounds

Overall Cost of an Extension

As of last year, the estimated cost of a single-storey extension outside of London fell anywhere between £1,200 and £1,500 per metre square. In London and the South East, the same work could cost you between £1,500 and £2,000 per metre square.

Overall, this could cost you a minimum of £40,000 at the lower bracket, and £65,000 in London, at the upper bracket.

Two-storey extensions, unsurprisingly, can cost much more. As a general rule, adding 50% of the cost of a single-storey extension will give you the approximate price of a double storey extension, built to your standards and desired finish.

Add 10% for professional fees and VAT, and for the example mentioned before, a double storey extension could cost you upwards of £76,000.

In London and the South East, expect this price to rise by an additional 30-50%.

Garage conversions, on the other hand, tend to be far cheaper. This is because instead of constructing an entirely new structure from the ground up, you’re building onto an existing structure.

If you budget correctly, and work goes to plan, your new garage conversion may cost as little as £10,000. Extending a garage or building on top of it, however, brings you back into single-storey extension territory, and will certainly cost more if the existing garage is structurally unsound.

Extension Cost FAQ 

Why would someone want an extension?

Often, extensions are built for one of two reasons. Either;

  • The homeowner wants more space, or,
  • The homeowner wants to add value to their property.

Depending on the extent of your extension project, the cost will vary greatly, but it can increase the value of your home by up to 5-10%. This makes the extension a cost-effective investment, especially if you’re in the process of renovating your home in order to sell it in the near future.

What are the disadvantages of building an extension?

Though the advantages of building an extension are numerous, you may find there are drawbacks, too.

When building a rear extension, you may lose valuable garden or patio space. Additionally, if the extension is built without consulting your neighbours, the design or size could cause conflict between yourself and those who live adjacent to you.

Avoid this by involving neighbours in the design process so they can voice any concerns they may have regarding the proposed build. Sorting out concerns prior to building is preferable to having them file a complaint when you apply for planning permission.

Are there alternatives to building an extension?

You may wish to convert an existing loft into an additional living space, instead of building an extension. Similarly, converting an existing garage (instead of extending it, or building on top) can increase the value of your property for a lower overall cost.

Find the cost of a garage conversion here and the cost of a loft conversion here.

[note: link to articles we have already written on these topics]

What are the main benefits of building an extension?

When building an extension, you benefit from a number of advantages.

Firstly, the cost of an extension is far less than the cost of moving to a new house. Often, households which start to outgrow their existing properties look at moving to a new house as a space-creating solution, but this costs them far more in the long run. Building an extension is a sizable investment but it is an economic choice in comparison to moving to a new property altogether.

Additionally, building an extension allows for personalisation during construction – this is something that moving to a new property cannot offer. When extending your home with an extension, you benefit from overseeing the design, size, and even the contractors themselves.

Finally, extending your home means you can increase the value of your property. An extension completed to a high standard can add value to your home, often by 5-10% at point-of-sale. Accessing quotes through Tradesmen Prices ensures the work completed is done to an exceptionally high standard. Please see the form at the top of this page.

Will I need planning permission to build an extension?

Most double and multi-storey extensions will require planning permission, though you may wish to check for single storey builds, too.

You will need planning permission if the build itself:

  • Covers more than 50% of the land surrounding your home
  • Is towards a road
  • Is built from materials which differ from those used in the existing style of your house
  • Increases the overall height of the building
  • Will include a balcony or raised veranda
  • Extends more than six metres from the back wall of an attached house
  • Extends more than eight metres from the back wall of a detached house
  • Is taller than four metres
  • Is more than half the width of your house

Hiring a professional architect or designer to ensure you stay within the Permitted Development guidelines will save you time, money, and stress.

Should my extension builder be a member of anything (trade body, government registration scheme, etc?) What sort of questions should I be asking them?

Because an extension project requires the specialist skills of a number of different types of contractor, there are several bases you’ll need to cover when you’re making sure that they’re the right people for the job.

Type of Contractor Accreditation Questions to ask
Builder N/A Ask about their public liability insurance, and the timescale for bricklaying and foundation digging. You may also wish to enquire about how much they’re paying for supplies.
Electrician Check for their name on the Registered Competent Person Scheme. Can you provide me with a BS7671 test certificate once you’re done so I can pass Building Regulations?
Gas Engineer Check to see if they’re APHC or CIPHE registered, and if they’re working with gas, that they’re a Gas Safe Registered engineer. Where is the best place to put a heater or radiator?
Plumber Check to see if they’re APHC or CIPHE registered, or similar. How long will rerouting pipework take? Will we have access to running water in other parts of the house during this time?
Plasterer They may be a British Gypsum Certified Plasterer. How long will it take to skim the walls (if applicable)?

How long does it take to build an extension?

Depending on the extent of your extension project, and who is doing the work, the length of time taken to complete the work will vary.

When building an extension from the ground up, there are several steps involved. These may take up to two weeks each to complete. Often, delays occur which can halt work for an unexpected length of time, so it’s crucial to ensure communication between yourself and your contractors is effective and timely in order to avoid this happening.

The typical extension process is as follows:

  • Week 1 – Preparation
  • Week 2 – Foundations and Groundwork
  • Week 3 – Ground and Low-Level Work
  • Week 5-6 – Walls (Internal and External)
  • Week 7 – Building of Roof Structure
  • Week 8 – Laying of Roof Covering
  • Week 9-10 – Windows, Doors, Guttering, and Render
  • Week 11-12 – Knocking Through Existing Wall and Plastering
  • Week 13 – Final Work E.g. Flooring, Electrics, Plumbing, Painting and Decorating
  • Week 14 – Garden Work and Finishing

An extension of any size will usually take a minimum of three months to complete and will cause a disruption to your home life, so be sure to consider this when deciding whether to have one built.

Why should I get extension quotes from Tradesmen Prices?

Building an extension requires a high level of accuracy in the design and construction process – after all, it will form a new part of your home which you’ll likely be using every day. Because of this, it’s important to find contractors who will complete the work necessary to the highest standards possible without having to pay more than you need to.

Shopping around for contractors means you’ll be able to access the best prices possible for your extension, but it can be exhausting having to manually trawl through quote after quote.

By using the form below, you can receive 3-4 quotes from reliable, honest tradespeople who will be able to undertake the work involved in constructing your extension.

Tradesmen Prices checks the credibility of these contractors before we allow them to receive quote requests through us, so you can rest assured you’ll be put in touch with verified local tradespeople, whose quotes you can compare against each other to find the best price possible.

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