How Much Does House Extension Cost

Maybe you have considered adding extra space to your London home but thought it was perhaps a bit too expensive. While the average price of a house in Greater London is currently around £5,000 per square metre, extending your London home will cost between £2,000 - £4,000 per square metre depending on the type of the project, the materials used, and the quality of the finish. 

Calculating the cost of extending a house

The big question is - how much does an extension cost and how cost is calculated? The overall cost of an extension depends on multiple factors such as:

  • Size of a building project
  • Extension type
  • Quality of materials and products used to build an extension
  • Average cost of house extension per m2 in your area
  • Construction labour costs
  • Complexity of designing and building
  • Your preferences and desired features
  • The current condition of your property
  • Soil type
  • Additional costs

It’s a good idea to take advantage of an online extension cost calculator. When using the calculator, you need to provide the answers to many important questions.  Do you want to have double or single storey extension built? Would you like to build a kitchen or bathroom extension? Are you interested in side or rear extension? Will you use basic quality, standard or premium quality building services?  

The cost of an extension depends on your answers. The tool will make all the necessary calculations based on the information provided as well as extension prices in your area and then give you the overall cost of a project (cost per m2).

Extension cost calculation will make it clear for you how much you’ll have to spend to build a dream extension. But most importantly, this will help you set the budget. If you would like to figure out how much exactly your extension will cost, ask builders to provide you with quotes. In most cases, property conversion specialists provide their customers with service quotes for free. 

Popular Types of House Extensions in the United Kingdom

Wrap around house extension

Home extension cost largely depends on the type of an extension project. There are different types of home extensions. Basement extensions, bathroom extensions, kitchen extensions and attic conversions are very popular in London. Many property owners also give a preference to side extension as well.   

For example, a Velux loft conversion will be cheaper than a one or two storey house extension, as it requires a lot less structural changes. Traditional masonry (An outer brick wall tied to an inner concrete leaf with an insulated cavity in between the two) will cost more than a modular timber structure but should last much longer. You might be happy with a kitchen from Ikea, or you might choose custom-made hand painted solid wood units, adding to the cost.

If you don't need more space, you could consider renting the extra room to benefit from your  home extension. A Dormer Loft conversion could turn your unused attic into a double bedroom. A single-storey house extension could accommodate a small studio. And a two-storey extension might be enough to create a one-bedroom flat, earning you up to £1000 a month.

Don’t forget about extra costs

Building projects have to comply with building regulations. You have to deal with different specialists during the construction projects. During the design and building process, you’ll need to cooperate with such professionals as an architect, surveyor, structural engineer and a project manager. Each specialist has his/her own responsibilities during the building process.

How long it takes to get the extension done

Getting an extra space in your home can take a long time. It consists of three main stages such as design, planning and building works. On average, for a loft conversion it takes about 4 - 8 weeks to get the extension done and about 10-12 weeks for a kitchen extension.

An architect prepares building plans and drawing for a construction project. When an architect finishes his/her job, the planning application is submitted to the local authority. An architect helps a homeowner get a planning permission for an extension construction project. Keep in mind that you will have to pay about 7% - 15% of the entire project costs for the job of an architect.

A surveyor prepares site for construction. A surveyor makes all the necessary measurements and determines heights and levels for an extension project. Builders work with the data provided by a surveyor. If you hire an architect in London, expect to pay about £600 - £1,400 for measured survey of an existing property and about £400 - £800 for topographical site survey.

A structural engineer is responsible for determining sizes and creating specifications for different extension’s components. It’s worth noting that the price of structural engineer’s services largely depends on the project’s size. Fees range between £400 - £4,000.

A project manager does different types of organizational tasks. Project manager’s responsibilities include: supporting a client during the construction process, helping a client make decisions, purchasing all the necessary building materials for a project, scheduling construction jobs and other types of project manage works. Plus, you’ll have to get building control checks done. So, you must be prepared to pay local authority fees. The additional costs include fees for submitting plans, planning application fees, building regulation fees as well as site insurance.

The prices on site insurance for extensions range between £500 and £800It costs £172 to submit an application for extensions and conversions of small size. Building regulations fees for a small single storey extension project varies between £500 and £1,000

And of course, these types of professional services will add to an extension building project costs. So, you must be prepared to spend about 5% - 15% of the total build cost on the additional extension costs. It’s important to note that all these service prices don’t include VAT. Keep in mind that up to 20% of the budget will be spent on VAT.

Cost of different types of extensions and conversions

    To give you an idea of how much money you will need to complete your project, we have listed different types of home extension and conversion projects sorted from the cheapest to the most expensive:

  1. Garage conversion
  2. Roof light loft conversion
  3. Dormer loft conversion
  4. Single storey extension
  5. Double storey extension
  6. Basement conversion
Double bedroom in a Velux roof light loft conversion

A garage conversion is an easy way to add an extra room such as an office or a gym to your house. If your garage has double-skin brickwork, you might be able to convert it for as little as £25,000. If the brickwork is single-skin, more advanced insulation will be required, adding to the cost.
A roof light conversion, often called Velux as these are the windows commonly used in this type of project, is an affordable way of adding an extra bedroom to your home. You will need to build a staircase, reinforce the floor, and add a couple of skylight windows. The cost starts from £30,000.
Dormer loft conversions are around forty per cent more expensive than Velux. With this type of project, the back (or sometimes the front) of the roof will be replaced with a flat roof dormer, creating a box-like extension with vertical windows, giving you more headspace.

Single storey rear house extension in London

A single-storey extension can connect your home with the garden. There are three types of one storey extensions available: a rear home extension, side return, and a wrap-around extension. You could use your extra space to create a large kitchen with a kitchen island or a cosy family lounge. The cost of this type of project will start from £40,000. 

Building these types of extensions may require a planning permission. Initially, you’ll work with an architect who will prepare the plans for your project according to building regulations. Then, plans have to be submitted to the local planning department and you’ll have to wait for their decision. If your plans have been approved, the building process can be started. First, you need to find a reliable building company. Give a preference to a construction firm who have done a similar job before and can meet your budget.

 types of double storey house extension

Also, it makes a lot of sense to ask builders how much does an extension cost in advance. The vast majority of building companies provide customers with quotes for their projects and inform them about extension costs in advance. So, get quotes and compare them to the average price of building the similar type of constructions in your area. 

The cost of a double-storey house extension will be about 50% - 75% more than a single-storey one. Two-storey extensions require more work and, therefore, additional costs. The size of work increases. Ground floor work has to be done. Plus, some changes have to be made to the roof structure. In this case, builders have to work at height. It’s important to note that these types of work are more labour intensive. Bear in mind that you’ll need to install new doors in your living environment.

Basement extension in London

You may also decide to have your old doors replaced with new ones during the renovation process. And of course, all of this will add to the price of a project. On the other hand, you will be getting twice as much space, making it a better value.
A two-storey extension can be added onto the back or the side of the property. In both cases, your floor area will increase and you’ll get the additional floor space. 

Building a basement is the most expensive type of extension project, with the price starting at around £5,000 per square metre. This is mainly due to the cost of excavation and underpinning the foundations of the existing house.
For this reason, basements are most commonly added to homes in popular London locations such as Kensington and Hampstead, where house prices can reach up to £20,000 per square metre. If you live in a prime location and have limited garden space and existing loft conversion already in place, you would definitely benefit from expanding your home below the ground.

Kitchen extension with Blue and grey shaker

Kitchen and bathroom extensions are one of the best ways to extend a building. However, this type of construction projects are very specific. It’s important to know that such an extension has to meet the strict requirements in order to comply with the building regulations.
Plumbing needs to be done to provide cold water as well as hot water in the room.
Also, you have to remove the wastewater out of a property. Whether you are building a bathroom or kitchen extension, it has to be connected to either existing drains or new ones. So, you must be prepared that plumbing and drainage works will increase your extension costs.

Clearly, the cost of a property renovation will increase if you choose to have a bathroom or a kitchen installed in your house.

white bathroom

The price of adding a bathroom or a kitchen will depend on the following two factors: the way you decide to build the construction and the quality of building materials that you’ve selected. 

On average, it will cost you about £3,000 - £20,000 to have a new bathroom installed and about £7,000 - £45,000 to have a new kitchen installed in your house.

Contact construction companies to find out the exact price quotes on building kitchens and bathrooms. It’s worth noting that some companies provide a pricing guide on their website.
So, customers have an opportunity to analyze the pricing guide carefully before getting the exact service quotes.

How much does it cost to build an extension

It’s also important to note that the construction costs depend a lot on such two factors as the size of an extension and the amount of building work that needs to be done.Clearly, the cost of small extension is lower comparing to the cost of big extension.There are small, medium and large extensions. Small projects cost about £40,000 - £55,000. Medium projects cost about £50,000 - £120,000. Large projects can cost between £85,000 - £300,000.

What is the cost of building an extension per square meter?

The average cost of building an extension per square meter is £2,000-£3,500 outside of London and £2,500-£4,000 in London

Take control of your building costs

When adding extensions to a house, it’s incredibly important for homeowners to get renovation expenses under control.
It’s possible to save money on renovation. However, you’ll be able to reduce your construction costs only if you take into account all things and get ready for the process seriously.

Plan everything carefully. It’s recommended to take some time to do the research and analyze the situation carefully. It would be better to calculate all the costs with VAT and set the budget in advance.

You need to know what works need to be done, the price of each service as well as the materials that you’ll use. Do you want to have underfloor heating installed in your house? Would you like to get ground floor renovated? Are you interested in replacing doors in your home? Or, maybe, you have some other preferences?

Discuss all of your needs with your builders to get the correct quotes! It’s a good idea to get several quotes for the same work. Then, you should compare quotes. Explore all of your options carefully and make the best possible choice!

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