Currently having an extension built. Demolised a garage adjoining a party wall. Building an extension away from party wall but need to make party wall good.
Excavated the entire site for single ground level. Extension now built and now need to address the party wall (render, etc.).
However, due to original garage floor being excavated Neighbour is concerned about frost damage to foundation strip on party wall. What is the building regulation for how deep below ground the foundation strip should be for frost protection. Their foundation strip for the party wall is about 300mm (dug an inspection hole) below ground level. They claim a builder has advised them the regulation is 900mm below ground level.
From your description you should have served your neighbour with a Party Wall Agreement.
If the party wall is a garden wall it may have been built with foundations at a minimum depth and no regulations would have applied.
If its a house wall then the foundation depth would have been controlled under BRegs but it depends how old the house is. Many 19th century houses did not have foundations as we know them.
If you were building a house now you would be expected to have the foundations at 0.9m but in clay or made-up ground they would be deeper and wider.
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