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[H.G.Muller]

Always store the true depth to which a position is searched in the hash. When you retrieve it, you can judge if this depth is sufficient for the depth you want now, taking the extensions into account. You don't care if the entry in the hash was made in an extended 5-ply search or a non-extended 6-ply search, it is the same search and so it should lead to an identical entry.

If you worry about some sub-trees being searched deeper in some cases, you would really have to store that information in the hash table. Logically it is part of the depth information. A stored depth (draft) of 6 with extensions suppressed should not be able to satisfy a request for a search of depth 6 with extensions switched on ('depth 6+'), but the latter search should not be able to satisfy a request for a 7-ply search without extensions. It is really an intermediate depth.