The subject of what is, or is not, a dwelling (for Council Tax purposes) is a 'can of worms'. I know, as I used to work with this sort of stuff. If something is a dwelling, then it gets 'banded'- that is to say, entered in the Council Tax valuation list. The 'person' that does the banding is the 'Listing Officer', who is an employee of the 'Valuation Office Agency' [VOA], which is part of Her Majesty's Revenue and Customs[HMRC]. So, the 'banding' is nothing to do with local Councils.
One of the ways in which something can be classed as a dwelling is if it constitutes a 'self-contained unit'. What is, or is not, a 'self-contained unit', depends upon the facts of the situation. The test is a 'bricks and mortar' test - in other words, is what is physically there, a 'self-contained unit'? What is/or was, in the mind of the person who did the construction is irrelevant (the High Court has commented in decisions that such a person might, in any event, not be around to ask, depending upon how long ago the construction was carried out).
I don’t know that I would be so bold as to state (i) that the fact that an interconnecting door exists between an ‘extension’ and the ‘original property’ means that the two could never be classed as separate ‘self-contained units’ and therefore attract their own separate bandings in the Council Tax valuation list, or, (ii) that two boilers, one in an ‘original property’ and one in an ‘extension’ would always result in separate valuation list entries. As previously stated, it is a ’bricks and mortar’ test, on the particular facts of each case. The last I knew, there were at least around half a dozen High Court decisions on this sort of situation – colloquially referred to as ‘granny annexes’.
The best people to speak to about this are the Valuation Office Agency, not the council. These are the contact details:
The Listing Officer / Valuation Officer
Eastgate House
42 Eastgate
Leeds
West Yorkshire
LS2 7JL
Tel:08456021507
Fax: 0113 3884602/3 (There is also an email address , which this site would not let me post.)
There is more reading, if anybody is interested, on the VOA website, in their Council Tax Manual, particularly Practice Note 5: Disaggregation of Dwellings. I tried to post a link, but this site won't let me.
Be aware, though, that this is just the VOA’s ‘take’ on the legislation and case law – Surveyors in private practice might take a different view.
Hope this helps.
Cheers,
Jez Wilkins.