What a trial hole tells you, and why it pays for itself
Foundation design is the largest genuinely unknown cost on most domestic extensions, and a trial hole is the cheapest way to remove that uncertainty. The exercise is simple: a small excavation, usually hand-dug for safety and to avoid damaging services, taken at or near the position of the proposed foundation and deep enough to expose what the foundation will bear on. What it reveals is worth listing, because each item changes a decision. The depth and nature of topsoil and any made ground tells you how far down competent bearing material lies. The material below it — clay, gravel, sand or a mixture — determines the bearing capacity and whether seasonal movement is a factor. The presence and depth of groundwater tells the contractor whether the excavation will need dewatering and the designer whether the construction needs to tolerate a wet environment. And the depth, width and condition of the existing house’s foundation is essential information in its own right, because a new foundation adjacent to a shallow Victorian one has to be designed so that excavating beside it does not undermine it, which is also the trigger for a party wall notice under the three-metre rule. Set against the cost of a project, this is a very small item, and it converts an allowance in a builder’s quotation into a designed element. The alternative — designing on assumption and discovering the truth when the machine is on site — produces exactly the variations that make people distrust construction budgets: the foundation goes deeper than allowed for, the programme extends, and the additional cost arrives at the worst possible moment. Where trees are nearby the case is stronger still, because the interaction between tree roots and shrinkable soil is what sets the depth, and that calculation needs to know what the soil actually is rather than what the neighbourhood usually has.
