"HMO" means a house in multiple occupation as defined by sections 254 to 259,
"Part 3 house" means a house to which Part 3 of this Act applies (see section 79(2)),
For the purposes of this Part "mortgage" includes a charge or lien, and "mortgagee" is to be read accordingly.
The appropriate national authority may by regulations provide for—
to have effect in relation to a section 257 HMO with such modifications as are prescribed by the regulations.
A "section 257 HMO" is an HMO which is a converted block of flats to which section 257 applies.