Average cost of a house extension in London

House renovations in London have an array of benefits associated with them. You are customising your home to suit your individual style and requirements. This is invaluable and can make day to day life much easier. The other advantage focuses on value from a financial perspective. Any sensible homeowner will want to add value to their property, right? It would be a bad investment to spend tens of thousands of pounds and to see no return on investment within the property value. The good news is, value is all in the figures. It’s fact. What did you buy the property for, how much did the works cost, and has it increased my property value? 

Here at Home Tales, we understand that budget is the number one concern for most of our clients. For most homeowners, the budget is capped. There is an end to how much you would like to spend. Budget is also important if you are looking for a return on investment, because the more you spend, the shorter the margin on your return. To help plan your budget, we thought it would be useful to offer some guidance on costs. This blog will be particularly useful if you are in the early stages of considering a home extension, and you want to see if it’s financially feasible before delving deeper into the process. 

Ground floor extension – side return extension 

A side return extension can only be done if you have a side return to extend into. Some properties do not have them and are completely flat to the rear. If you do have a side return, you can extend to the side and encompass the area into your room. In this case the length of the room doesn’t change, but it becomes much wider. Most side returns in London are around 6m in length and 1.5m in width. Most London properties are approximately 4 to 5m in total width, so with the extension, this gives you a finished room size of around 4.5 wide and 6m in length. This is easily large enough to house a large kitchen and dining area. From a cost perspective, you can expect an extension of this size to cost approximately £55,000 to £75,000. 

Ground floor extension – rear extension 

A rear extension is often done if the property doesn’t have a side return and is flat to the rear. The most common rear extensions we undertake here at Home Tales are approximately 3 to 4m in length. A typical London property is 4 to 5m in width, so the additional SQM area to the property will be 12-20 SQM. Most of our clients want to add this onto the existing rear room within their property, to create a wonderfully large open plan room. An extension of this size will cost approximately £60,000 to £70,000. 

Ground floor extension – wraparound extension 

A wraparound extension uses a combination of both a side return extension and a rear extension. It maximises the internal space of your property, adding a considerable amount of floor area to your property. As it’s the largest of the three types of extension, it is also the most expensive. That being said, the more space you add, the more cost efficient it becomes. You can expect a wraparound extension of 8m in depth and 5m in width to cost around £70,000 to £80,000. 

Loft conversion – dormer extension 

Loft conversions are incredibly popular in London. If you want to add bedrooms to your property, they are by far the most cost-efficient option. A dormer extension will open up the rear pitch of your roof and bring the roofline flat. This gives you a usable room with a decent head height. We will typically add some Velux windows into the front pitch to increase natural light flow within the room. With a dormer extension, you can generally create a decent size double bedroom and en-suite bathroom. This kind of extension on a 4 to 5m wide property will cost around £35,000 to £45,000. 

Loft conversion – l-shape dormer extension 

If you have a two-storey existing outrigger, then you might want to consider an l-shape dormer extension. This includes the dormer extension mentioned above and adds a second dormer to the rear of the property, which sits on-top of the two-storey outrigger. This generally enables the creation of a second bedroom or a spacious bathroom. An l-shape dormer extension will cost around £45,000 to £55,000.

It’s important to remember that each property is unique. The sizes and associated costs mentioned above are just generalisations and are there to offer a guideline on costs. If you are keen to understand more about how much value an extension can add to your property, we have a blog article that answers just that. If you would like a cost specific to your property and project, call our team today on 0207 043 2378 or email us at hello@hometales.co.uk. Alternatively, you can book a consultation directly via our live booking page on our website. Our consultations allow us to learn more about your project which enable us to issue you with a quote for the works. We also take the opportunity to explain more about how we work and the overall design and build process. 


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