Going six feet under?

Building a basement for an existing house can be a great investment for the owners, but there are plenty of issues that have to be handled with care if the project is to succeed.

The profile of the existing foundations and the nature of the ground at basement depth must be established. New footings will have to be designed to support the existing structure as well as any vertical loads from adjacent structures, roads and retained earth.

Water is another major consideration. What are the ground-water levels? Do any underground services need repair? Would the slab be able to resist uplift pressure from the ground water table?

All the above should be sorted out at the concept stage, before any other parties are involved, to establish whether the provision of a basement is feasible. Trial pits exposing the existing foundations and to the depth of the proposed basement should be carried out. Those should be left open to measure the water table during wet and dry seasons. This will help establishing the best structural solution for the basement.

Then the design team need to agree on what waterproofing system will be used? Very often access to the back of the retaining wall is not possible, therefore a cavity drainage system to the face of the existing or new wall is the best solution. The water pressure will then be drained within a gutter at the bottom of the cavity and pumped out of the basement. A secondary masonry wall internally will be required and a plant room for the sump and pumps needs to be allowed for.

The other thing to deal with if there’s an adjoining property is a Party Wall Award detailing the proposed works, the sequence of construction for the underpinning and the design of any temporary propping during the construction.  Cracking to the adjacent properties is to expected (either due to local stress redistribution or due to construction vibration) and cracks repairs and repainting of the adjacent properties’ walls are to be allowed for in the cost of the project.

Speaking of walls, don’t forget that there’s an option of creating a secondary retaining wall, offset from the external wall, so that light can flood into the new basement.

It’s vital that all surveys, slab design, work sequencing, planning of temporary works, inspections and so forth are conducted by people who know exactly what they’re doing. Equally, the construction work has to be carried out by a competent contractor who has done similar work before and has full insurance cover.

Related project: Cranbrook Road