I think so, yes.
Since they tax by the square footage in England, his publicizing that good fortune enlarged his house is probably going to result in a large tax bill every year.
If he kept quiet, he could have renovated that room and just kept it secrat by a swinging bookcase.
By floor space?
The Doctor: tax avoider extraordinaire
They do not tax by square footage. They have tax bands for various valuations of house for council tax purposes, but the bands are broad and the council tax is not very high. The tax is ad personam rather than ad rem.