We all know that that there are substantially different SDLT rates for a dwelling (where the top rate may be 15%) as opposed to a commercial building (where the top rate is 5%). So it is crucial to understand what is meant by a dwelling.
There are subtly different interpretations between the different taxes. I think a bathroom, a living/eating area and privacy are core ingredients. Into the mix, HMRC in its SDLT manual suggests that the living/sleeping area (or areas) would normally each have a window, but that had been a debatable point for about 12 months.
While the planning inspector in a case last year did agree that "living without a window would not be a positive living environment", nevertheless he allowed the appeal. The council had refused permission on the grounds that the quality of the accommodation was so poor that the units could not be considered "dwellings".
Perhaps after months of lock-down it is unsurprising that this decision has effectively been reversed and, from 1 August 2020, we have rules that enshrine "the need for adequate natural light in all habitable rooms" (https://www.legislation.gov.uk/uksi/2020/632/contents/made). Amongst other issues, taxpayers may need to reconsider their claims for multiple dwellings relief (MDR).
...when offices are converted into flats, the rooms must have "adequate natural light", under new permitted development right regulations.