Housing Act 2004 - Section 258
HMOs: persons not forming a single household
This section sets out when persons are to be regarded as not forming a single household for the purposes of section 254
(2) persons are to be regarded as not forming a single household unless-
a - they are all members of the same family, or
b - their circumstances are circumstances of a description specified for the purposes of this section in regulations made by the appropriate national authority.
(3) For the purpose of subsection 2(a) a person is a member of the same family as another
person if-
a - those persons are married to each other or live together a husband and wife (or an equivalent relationship in the case of persons of the same sex);
b - one of them is a relative of the other, or
c - one of them is, or is a relative of, one member of a couple and the other is a relative of the other member of the couple
(4) For the purposes-
a - a 2couple” means two persons who are married to each other or otherwise fall within subsection (3)(a);
b - “relative” means parent, grandparent, child, grandchild, brother, sister, uncle, aunt, nephew, niece or cousin;
c - a relationship of a half-blood shall be treated as a relationship of the whole blood; and
d - the stepchild of a person shall be treated a his child
Occupied means 'lived in' (Silbers v Southwark LBC 1977). Therefore vacant houses cannot be HMOs under the legal definition.
So if the criteria fits, the property, by definition is an HMO.
Would this help also
http://www.landlordzone.co.uk/HMOs1.htm