The cost of constructing foundations is one of the biggest variables in a self-build or extension project, so anyone who wants to keep control of their budget and schedule needs to work out the most appropriate foundation system to use as early as possible.
Identifying the most cost-effective solution will depend largely on the ground conditions on site, including the type of soil, aggregate or rock and its key constituents. The best way to achieve this prior to starting work on site is to instruct a geotechnical study (see box, below) which involves digging or boring trial holes around the site of a proposed new building, principally to establish the load bearing capacity of the soil at differing depths. Samples are collected and tested for plasticity (shrinkability), pH levels, sulphates, moisture contact (seasonal water table) and other factors that will allow the foundation solution best suited to the site to be determined.
GEOTECHNICAL STUDY: A WISE IDEA
Neglecting to investigate the ground on site is one of the earliest and most costly mistakes a self-builder can make. The more cautious may wish to instruct a study prior to purchasing a site - if the vendor will consent - as ground conditions can influence the viability of a project and therefore the price worth paying for a site. A geotechnical study will cost from £600 plus VAT upwards depending on the size of the project.
A less invasive option is a desktop geotechnical study, which will cost from £500 plus VAT and will perform a host of record searches into geology, potential soil contamination from previous uses, flood risk and much more. Both options should be considered.
Infiltration tests can be conducted at the same time to assess soil permeability and the suitability of the site for soakaways for storm water and for drainage fields for off-mains sewage treatment. This is important for establishing your drainage strategy, which needs to be submitted at the planning application stage.
At an earlier stage you can undertake your own desktop feasibility review by calling the local authority building control department to ask the likely soil conditions around the site address and the foundation solution typically required in the area. Many local authorities produce a fact sheet on typical foundation solutions for different soil types commonly found in the area. You can also refer to the searchable maps of the British Geological Survey: https://bit.ly/2DGd7UO. Another useful resource is the Building Regulations Approved Document A: 2004 - see page 103.