The new guidance, found at SDLTM00415 (How many dwellings?) explains that HMRC will apply an "objective observer test" when deciding how many dwellings there are in a property purchase. It is this test which HMRC apply during compliance checks for transactions involving MDR.
HMRC will ask "how many dwellings an objective observer would consider that there are" in a particular transaction. Recent decisions such as Fiander and Brower  or Andrew and Tiffany Doe  (both of which ruled in favour of HMRC) highlight what HMRC consider to be the "correct approach" to ascertaining whether a transaction consists of one or more dwellings and therefore whether MDR should apply.
HMRC's updated guidance and the recent tribunal decisions again demonstrate HMRC's determination to challenge contentious MDR cases. Specialist SDLT advice should be sought at an early stage in any property transaction which includes an annex or additional "dwelling" to clarify whether MDR should apply.