asked Her Majesty's Government:

Whether a permanent tree house within the curtilage of a domestic dwelling is taken into account by the Valuation Office Agency when a property is valued for council tax. [HL6350]

In the case of a new dwelling, all features of the property that affect value are taken into account in the banding. The test is whether, in the open market, the presence of a tree house would have any material effect on the property's capital value: in general this is unlikely.

In the case of an existing dwelling, if the tree house was a later addition that added value, it would only be taken into account when the dwelling was subsequently sold, or let on a lease for more than seven years.