Tom,
I agree, but in a court ruling that I have but not with me (Civvy mentioned it in another post and I can not find that either) the Judge ruled that any part and any appurtenace of a property where the person can go is still part of a domestic premises. Now I think it could be argued that a common staircase is an appurtenance of a premises and therefore could stand outside of the RR(FS)O particularly if the propert is not counted as an HMO and would never be subject to any licensing conditions.
Hi Jokar
Be aware that the common parts of a HMO, licensed or not, are subject to the requirements of the RR(FS)O. As Kurnal already pointed out there is a difference in definitions and legislation in Scotland which I believe is where the case Civvy mentioned occurred.
The only grey area which seems to cause confusions are Shared Houses. According to the Lacors guide, the whole property is considered to be so similar to a single domestic dwelling that the RR(FS)O order wouldn't apply, however be aware that the term "shared house" has no legal definition, and it's status or meaning or legal standing has not been tested in court.
Local Authorities enforce standards within HMOs, and have the mechanisms to enforce fire precautions in the common and "domestic" parts of the HMO. Often the Fire Authority will act as advisors on behalf of the LA in this respect, where the RR(FS)O doesn't apply.