Assess the whole route to the ground
The check is not limited to the foundations. Existing walls, beams and earlier openings may be part of the load path. Planning Portal's guidance on additional loft loads highlights the need to consider old knock-through beams and the construction beneath the new work. The same question is central here: what will actually carry the new storey? [2]
Ask the engineer to mark the supporting elements on the existing plans and identify any information that remains unverified. Supply details of alterations, not only the drawings for the first extension.
Find the original information
Look for structural calculations, foundation drawings, inspection records and photographs taken during construction. Ask the engineer which records are useful and whether their assumptions can be checked against the building as it now stands.
Council building-control records are not necessarily open for unrestricted public viewing. Camden, for example, requires evidence of ownership or the owner's authority for access to relevant documents. Check the record-holding body's procedure rather than assuming that a planning-portal download contains the structural design. [3]
What investigation may be needed?
A trial pit can expose an existing footing and provide information about the adjacent shallow ground. [4]
Agree a targeted investigation brief before opening up the extension.
NHBC Foundation's 2023 review of foundation reuse identifies the importance of reliable records, condition, capacity, geometry and compatibility with the proposed loads. It presents reuse, supplementary support and new foundations as different possible outcomes, not a presumption that existing foundations must always be replaced. [1]
Compare the engineering options early
Ask the design team to compare retaining the existing support arrangement with any necessary strengthening or a revised design. Where underpinning is proposed, request the reason: Planning Portal notes that increasing building loads can be a reason to strengthen existing foundations. [5]
Use the option review to consider disruption as well as the visible addition. Ask where supports would appear, which rooms would be opened up, what investigation remains and which construction assumptions affect the quotation. Do not commit to a fixed build programme while the supporting structure is still unresolved.
Approval still covers the complete alteration
Adding a storey requires consideration of building regulations beyond structural capacity, including fire safety and the means of access and escape. Planning Portal's upward-extension guidance confirms that building-regulations approval is required for this type of building work. [6]
A favourable structural assessment is not a planning permission. Treat the final feasibility decision as a coordinated one: a viable structure, an acceptable layout and an identified approval route. Retain the engineer's conclusion and any qualifications with the final drawings so that the construction team works from the assessed proposal, not an earlier concept.