Home Extension Guide to Costs and Benefits

Wrap around house extension

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 £1,500 - £4,000 per square metre depending on the type of the project, the materials used, and the quality of the finish. 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.

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 £20,000. 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 £25,000.

Single storey rear house extension in London

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.


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.

Basement extension in London

The cost of a double-storey house extension will be about fifty per cent more than a single-storey. However, 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.


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.

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