NI Building Regulations look like they can have greater scope on alterations than the E&W ones so could be interpreted to allow this requirement(rightly or wrongly). Besides if they are concerned they could just pass it over to NIFRS who would simply use the The Fire Safety Regulations (Northern Ireland) 2010, I'm aware of cases where FRS have take action against unprotected basements in existing premises.
Application to alterations and extensions
7.?(1) Subject to the provisions of regulation 4, Parts A to L and Parts R and V shall apply to?
(a) a structural alteration or extension of an existing building; and
(b) the existing building as affected by that alteration or extension to the extent (subject to the
provisions of regulation
of prohibiting any alteration or extension which would cause a
new or greater contravention of any regulation.
(2) In applying the regulation under paragraph (1)(a), the alteration or extension shall be treated
as if it were part of a building being newly erected identical to and to be used for the same
purposes as the building as altered or extended.
(3) In determining for the purposes of paragraph (1)(b) whether the alteration or extension
would cause a new or greater contravention of any regulation, the following provisions shall
apply?
(a) the regulations shall be applied in each of the following ways?
(i) to the building as altered or extended treated as if it were being newly erected in its
proposed form for the purposes for which it will be used when altered or extended;
and
(ii) to the existing building treated as if it were being newly erected in its existing form
but for the purposes for which it will be used when altered or extended; and
(b) the alteration or extension shall be regarded as being such as would cause a new or
greater contravention if (when the regulations are applied as directed in sub-paragraph
(a)) the building as altered or extended?
(i) contravenes any regulation which does not apply to the existing building;
(ii) contravenes any regulation which is satisfied by the existing building; or
(iii) contravenes to a greater extent any regulation which is contravened by the existing
building.