Start with the location plan
First locate the tests relative to the proposed extension. Check the address, investigation date and reference used for levels. A test beside the street may not describe the conditions in the back garden. Forward the complete report, including its plan and notes. A screenshot of one graph often omits the very context needed to assess the information properly.
What do the plotted lines show?
A cone penetration test measures, among other things, the resistance encountered as a cone is pushed into the ground. Additional measurements depend on the equipment used. These lines represent readings; interpreting soil layers and design parameters requires expertise. Do not select a pile length by locating the largest peak. Foundation type, building loads and potential movements also influence the advice.
What should you ask the adviser?
Ask whether the available test locations adequately represent the proposal and what further information is needed. Record who prepares the geotechnical advice and how the structural engineer uses it. Changing from a lightweight extension to a heavier design may change the investigation question. Keep the investigation report together with the version of the building proposal on which the advice was based.
Practical checklist
- Keep the complete test record
- Check test positions and level reference
- Share the current proposal
- Ask for specific foundation advice
A nearby test is not automatically sufficient for your own project.
Preparing your own project
Would you like this assessed for your home? See our information about foundations. It explains which information is relevant to that brief. The project check helps you prepare your enquiry.
Unsure about a term? The structural glossary provides plain-language explanations and technical context. To understand how the documents are organised, explore our fictional example project .
Background and sources
Sources checked on 3 October 2026. This guide is not a project-specific assessment.
Editorial guide • updated 3 October 2026. General information; not a project-specific assessment.