If a basement conversion is anything more than cosmetic, it will involve planning regulations and have insurance implications. Navigating the relevant building regulations and insurance requirements is a job best overseen by a professional. Gaining approvals usually requires preparation of a structural design and sometimes geological tests.
A basement construction in London needs to show compliance with the Construction, Design & Management Regulations of 2015, and if your property is adjacent or in proximity to other buildings (and where in London isn’t?) you should comply with the Party Wall Act 1996 by reaching an agreement with neighbouring owners.
Regulations require you to consider lighting, drainage, energy efficiency and a host of other issues but one of the hardest to assess is the strength required of foundations and supporting walls, especially at the points where sewers, drains and services penetrate the building.
The good news
There are many new methods for stabilising foundations. Some allow floors and walls to be strengthened or underpinned with relatively little excavation and disruption to existing living space.
The simplest, and still most common method, is to remove loose material and replace it with a layer of concrete. More sophisticated techniques include; cantilever piles and beams, piled rafts, screw piles and brackets, soil compaction, soil strengthening injections and jet grouting.
For new builds, an alternative to piling is vibro-compaction. A vibrator, sometimes helped by water jetting, is used to compact loose materials to a depth of as much as 29 m below the building. Vibro-replacement is a similar technique that transfers the building’s weight to deeper stable levels.
Ground goes up as well as down
In London, we have deep layers of clay that contract as they dry and expand when they’re wet. A period of heavy rainfall after a period of drought can raise the ground by 150 mm or more. Leaking pipes near your home can have the same effect, cracking walls, membranes and foundations. If the water freezes, it expands even more.
If subsidence is due to drying soils, rehydration is one simple option. When heave is the problem, cellular structures are installed under foundations or floor slabs to relieve the upward pressures.
While many people fear that nearby trees will crack pipes and walls, felling them causes even more problems. Trees help remove excess groundwater that might otherwise cause the ground to swell. In addition, once felled, their roots leave cavities in the ground which could promote subsidence or let in even more water, causing heave.
Ironically, excavating deep new basements can also cause the ground to rise because the weight of material removed allows compressed layers beneath to expand. In major basement constructions, appraisal of how the ground will react to proposed works is needed in advance. Test boreholes can be made to confirm the geological conditions.
Settlement cracks in both old and new buildings aren’t unusual and usually require no more than minor repointing, but if remedial works are needed, an experienced builder or surveyor will determine what kind of ground movement is taking place. Collapsing drains, culverts, and forgotten tunnels and well shafts must also be ruled out.