Deciding whether it’s best to convert a loft, or add a retrofit basement to a property can be a difficult decision to make. To make the choice a little easier, we’ve done some digging and have identified the advantages and disadvantages of both loft conversions and retrofit basements based on cost, time, functionality and size.

Cost of Basement VS Loft Conversion

Depending on what the extension will be used for, costs can vary greatly – regardless of whether it’s a loft conversion or retrofit basement. Of course, indulgent extras like swimming pools or dormer windows will bump up the price, but it helps to understand the basic pricing of each extension type before you get planning permission.

In general, a loft conversion is the less costly option, as the main shell is already present. Prices range from £10,000 for a basic conversion and Velux window, to £30,000 plus if you want to include structural changes like dormer windows.

Adding a basement under an existing property however, will cost around £2000 – £3000 + per per m² subject to the size, shape of the property and the extent of temporary support works required, bearing in mind that narrow properties will cost more than squared properties as well. Fit-out costs will run between £800 – £2000 per m² (dependant on the quality of finishes), so the final price will depend entirely on the size of the finished space and the finishes chosen.

Time for Basement or Loft Extension

While a loft conversion takes around 4-6 weeks, a retrofit basement needs more time for digging out the foundations, underpinning and proper waterproofing.

A reputable basement construction company will value engineer the project, to make the process as quick as possible without compromising on the quality of the work. Depending on the size of the project undertaken, you could have the finished basement in as little as 6-8 weeks.

Functionality Required for Home Addition

This is where the basement really is the ideal addition to a modest London home in need of extra space. While a loft conversion can function as a bedroom, study or spare room it is limited to the predefined shell of the property, meaning you cannot expand beyond the size of the existing property.

In contrast, the potential uses for a basement are limitless, as you are not confined by the existing boundaries of the property and can push beyond into garden space or deeper underground, creating more than one storey. Some of our basement projects have included gyms, cinemas, garages and even subterranean swimming pools.

Size of the Additional Space Required

As mentioned, the size of your loft conversion will depend entirely on the size of your house. In addition, the height of your new room will be limited by how low your roof is whereas with a basement, you can dig as deep as you like and extend below the garden if required.

The fact is, there is no right answer as to what the best choice is when it comes to loft versus basement extensions. In reality, it all depends on the existing property itself, the budget, the space required and the functionality of said space.

If you think that a basement would be the best way forward for your London property and would like some expert advice, Knowles have over 20 years of experience in the highly specialised field of basement construction on projects of all sizes and work with many top architects in the greater London area.

If you currently require a consultation for an upcoming basement project, get in touch with one of our experts.