Why your neighbour's foundations are not a design for your extension

Foundations transfer the loads from a building into the ground. A suitable solution depends on both sides of that relationship: what is being supported and what the ground can do. Two similar-looking extensions can therefore need different approaches.

The ground can change across a small site

A garden may contain natural soil, areas disturbed by previous building work, old foundations, filled features or buried services. The visible surface does not reveal all of that history.

Nor does a neighbour's excavation establish the conditions at your proposed building position. Their records can inform an assessment, but the engineer needs evidence relevant to your footprint and the proposed work.

Start by gathering what already exists: previous plans, survey information, records of extensions or drainage work, and any relevant information from the purchase of the property. Tell the engineer about features you remember being removed, such as an old outbuilding. An apparently unimportant detail may help explain a change found during investigation.

Avoid paying for a generic investigation without an agreed question. Ask the engineer what needs to be established and which investigation would provide useful evidence.

Trees are part of the assessment

Some clay soils shrink as they dry and swell as moisture returns. The amount of movement depends on their composition and moisture changes. Trees can influence moisture conditions, but the mere presence of a tree does not establish a foundation design or diagnose a defect. The British Geological Survey explains the relationship between clay, moisture and shrink–swell behaviour. BGS: clay shrink–swell.

Show the engineer relevant trees and hedges, including those beyond your boundary, and mention known removals or significant changes. An arboricultural assessment may be needed alongside the engineering advice.

Do not remove a tree simply because it appears close to the extension. Removal can have its own consequences, and tree protection or conservation controls may apply. Get the design and any tree advice coordinated before deciding what should happen in the garden.

The proposed building changes the question

A lightweight roof, masonry walls, concentrated loads from a steel beam and an additional storey do not all load the ground in the same way. The arrangement of the building matters as well as its overall size.

For example, a large opening in the rear wall can transfer loads to a smaller number of positions. Those support points need to be considered as part of the structural design. A plan showing a broad open space does not show every consequence for the foundations below it.

Approved Document A covers structural matters including foundations, walls, floors and roofs. The foundation design needs to work with the complete structure, rather than being selected independently from a standard-looking detail. GOV.UK: structural guidance.

If a future second storey is a genuine possibility, tell the designer now. Ask what can reasonably be designed for, what assumptions would be made and what would still require review later. No foundation detail can promise approval for a future scheme that has not been defined.

Existing buildings and drains complicate the edges

The junction with the house deserves attention. The new work may sit near older foundations whose depth and condition are initially unknown. Excavation also needs to be planned so that existing structures remain supported throughout the work.

Nearby drains and sewers may affect the available foundation positions and construction method. Their ownership and any required permissions should be established separately from the structural calculation.

Work close to a neighbouring building can also bring the Party Wall etc. Act into consideration. This is a separate assessment, with distance and depth both relevant to certain excavations. A foundation drawing does not, by itself, settle the notice requirements. GOV.UK: party walls and building work.

These are reasons to coordinate the design early. Moving a support after drainage or neighbouring foundations are understood can affect much more than the concrete quantity.

Know what is assumed in the price

Ask a tendering contractor to identify the foundation arrangement they have priced and any assumptions about ground, excavation, access, spoil removal and water in the excavation. Establish which items are fixed and which are allowances subject to what is found.

“Foundations included” is too vague if the design is still provisional. A useful quotation makes the basis visible so you can understand where the remaining uncertainty sits.

Ask what evidence could reduce that uncertainty before construction. Earlier investigation may make pricing more reliable, but it will not reveal every possible condition. Keep a suitable project contingency and an agreed process for assessing changes.

Plan for the moment the ground is opened

Before excavation starts, agree the inspection arrangements and who must be contacted if conditions differ from the design assumptions. The contractor should know the relevant hold points and how information will reach the engineer and building control body.

The response to unexpected ground should be an assessed decision. It should not be an informal instruction to dig deeper or pour more concrete without checking the consequences.

Keep the resulting records with the project documents, including approved changes and relevant inspection information. They may help explain the building to a future owner or designer.

When planning an extension with CR Design Services, share what you know about the site and what you hope the building may need to do in future. Architectural and structural decisions made together can identify the questions that need answering before the foundations are priced or built. Explore CR Design Services.

Guidance for homeowners in England. Reviewed 8 September 2026.