Extensions to buildings need their own foundations.
In the Netherlands, relevant areas have regulations to ensure that piles are deep enough to support any structures added onto houses or other buildings - these have developed over the years and are now much more stringent than in the past.

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Note added at 2 hrs 37 mins (2005-07-20 00:56:41 GMT)
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e.g. if you want to add a conservatory to your house, you have to have piles of a specific length and cross section hammered in onto which the foundation/floor can be laid.