An extension foundation is selected from the ground conditions, loads and relationship with the existing building. Start with available drawings and an assessment of the site. The floor area of the proposed extension alone cannot determine the appropriate foundation approach.
Which information about the existing house is available?
Look for original drawings, previous structural records and information about alterations to the rear elevation. Establish what is known about the original foundations and which details remain uncertain. Record visible symptoms such as cracks or sticking doors without assuming their cause.
Haarlem municipality operates a foundation information service, the funderingsloket. It does not replace project-specific assessment, but can help identify available records and useful next questions. Associate each document with the correct property and building phase so information from an earlier proposal is not mistaken for the completed work.
Which investigation addresses the design question?
A ground investigation such as a cone penetration test can provide information about soil conditions, but the question should be clear before evidence is collected. The design determines relevant loads and dimensions. Access for investigation may also matter in an established garden.
Have the appropriate specialist identify additional information and explain how results inform the proposal. Information from a nearby plot can provide background without necessarily being sufficient for your property. Keep conclusions with their supporting records so later decisions can be traced to the evidence actually obtained.
How do existing and new foundations interact?
The original house and extension may respond differently to loads and movement. Their junction therefore needs deliberate structural development. Ask how the chosen approach accounts for the existing building and what construction sequence follows.
Describing a foundation independently from the building above is insufficient: walls, floors and openings form part of the same system. Keep uncertainties about existing components visible until investigated. Marking an assumption provisional is more useful than presenting it as a confirmed basis for a quotation when nobody has verified it.
Which tasks belong in the foundation work package?
A foundation allowance may encompass several different operations. Specify the included preparation, excavation, construction and checks. Discuss access for equipment, removal of excavated material and protection of retained features.
Identify information required before work starts and who checks that encountered conditions match the design assumptions. Changes made during construction should be recorded. This leaves an understandable account of what was actually built and how the connection with the original house was developed, rather than only a drawing of an intention that may later have changed.