The den conversion is mentioned in Carol Ann Lee's book, on page 91 (2020 edition):
"At the end of February 1984, Jeremy bought a Vauxhall Astra for almost £6,350 on the N. & J. Bamber account. He sold his old Mini to a labourer helping builder Dennis Wager renovate White House Farm's scullery. Everyone used the back door as the main point of entry to the house and Nevill had decided to modernize [sic] the whole area. He got rid of the old fireplace and bread oven, installed a stairway to the first floor office where Barbara worked, and had a new office built for himself."
This seems to imply that the conversion was prior to P.C. Dryland's inspection in August 1984, but it's not made clear. The sale of the old Mini could have been long after Jeremy bought the new car, for instance.
Unfortunately, Carol Ann Lee doesn't provide sources for the information in this paragraph (I don't like the way she uses sources anyway, but it would help). I have a feeling that the relevant information is buried in one of the statements given by the relatives.