Foundations transfer building loads into the ground. The appropriate arrangement depends on ground conditions, the structural proposal and the original building. For an extension, floor area alone therefore cannot establish a suitable foundation solution.
How this term relates to your project
The foundation belongs to the complete load-carrying arrangement. Walls, floors and openings influence the forces reaching it. Ask what is known about the existing foundations and which assumptions need investigation. A neighbour’s drawing does not automatically justify the approach for your property.
Discuss the relationship between the original building and extension too. A quotation should identify the included investigation, excavation, construction and checks. Keep relevant records with the house documents so later alterations are assessed using information about the actual structure rather than the visible finishes alone. Distinguish a proposed design from documentation of completed work.