The main types
Strip foundations use a continuous strip of concrete beneath a wall, with foundation masonry built above. Trench fill uses substantially more concrete within the excavation, reducing the amount of masonry needed below ground. Both require suitable ground and a properly designed depth and width. [2]
A raft spreads loads through a designed foundation slab over a larger area. Piles transfer loads through deeper foundation elements, normally linked to the building by designed caps or ground beams. Their behaviour and suitability depend on the complete arrangement, not just on how deep they extend. [1]
Ask the engineer to identify the proposed type on a section through the building. This is more useful than accepting a quotation referring only to “standard foundations”.
What drives the choice?
A trial pit exposes shallow ground or existing foundations; boreholes can investigate deeper conditions. [3]
Ask which questions the proposed investigation will answer. Request a scope matched to the structure under consideration, rather than accepting a fixed investigation package without an explanation of its relevance.
Ask the design team to explain the principal constraint: weak or variable ground, tree-related movement, nearby structures, restricted access or the loads being introduced. Request the reason for rejecting simpler alternatives where specialist foundations are proposed.
Include the connecting structure
NHBC Foundation's 2023 climate-resilience report discusses foundation systems as a whole, including ground beams, excavation and concrete use. The design should not ignore a difficult connection between the foundation and the superstructure simply because a piling contractor has supplied a price per pile. [4]
For quotation comparison, request clarity about investigation, design, excavation, spoil removal, concrete or piling, connecting beams, temporary works, testing and reinstatement. These are comparison headings, not a claim that every item is required on every project.
Approval and inspections
England's structural requirements include safely transmitting loads to the ground and accounting for reasonably foreseeable ground movement. Approved Document A provides guidance, but a named foundation type does not demonstrate compliance without a suitable design. [5]
Agree inspection stages with building control before excavation begins. LABC's extension guidance identifies foundation inspections before concrete is poured, making it important to leave relevant work available for checking. [6]
Standards are not all the same thing
The NHBC Standards 2026 apply to NHBC-registered homes whose foundations begin on or after 1 January 2026. That applicability should not be confused with a statement that every private extension is governed by an NHBC warranty standard. [7]
Ask the engineer to state the design basis and any warranty requirements applicable to your project. The quoted foundation package should follow that basis and the actual investigation, not a generic depth or an assumption copied from the house next door.