basement extension under garden

How much will it cost for a basement extension under my garden?

If you have a set of existing rooms or a cellar space under your house, a standard basement conversion may be the most cost-effective option to make this space usable. However, to retrofit a basement by digging out underneath your property will cost significantly more. There is a third option; to create a basement extension under your garden. Whilst not as cheap as a standard conversion, it is cheaper than digging deep under the foundations and has a range of benefits including the ability to borrow natural light.

When it comes to pricing it up, however, the basement extension under garden cost can differ depending on a range of factors, chief of which is size. But because there’s no underpinning involved, you can generally work on a basement conversion under a garden costing half as much as digging directly under your property. That’s because there’s a significantly reduced risk of structural damage.

Basement extension under garden cost factors

There are a number of factors that can impact on the cost of any basement extension:

  • The only limit to the size of your basement is the size of your garden. A larger basement obviously costs more.
  • Light wells that provide plenty of natural light can add to the cost.
  • External access to your basement can add to the cost.
  • Access issues can bump up the cost.
  • A high water table is a key factor in pricing your excavation.
  • Certain types of soil will have a negative impact on the excavation of your under garden basement and can increase costs.
  • Diverting drainage and sewers can be a costly procedure if you need to do this.

Additional fees and costs

You’ll need to budget for:

  • Architect’s fees, which can be as much as 15% of the cost of the entire build.
  • Structural engineer’s fees which will depend on the overall size and complexity of the project
  • Planning applications aren’t always necessary, but research your local authority planning portal and budget for them just in case.
  • Survey fees for water table, soil and geology, trees and drains can be several hundred pounds each.

Is the cost of a basement conversion under my garden worth it?

Obviously it’s a subjective decision, but there are several benefits to this style of basement conversion that make it well worth considering. Apart from the opportunity to add a large living space, depending on your budget, garden basement conversions are lighter, faster to create and fresher than those directly under your property. There’s also a much lower structural risk to both your and your neighbour’s property.