And in Part M it says "The requirements of this Part do not apply to an extension of or material alteration of a dwelling".
And read your quote from Approved Document P (which is only guidance anyway, and not mandatory) again:
Accessibility
1.6 Wall-mounted socket-outlets, switches
and consumer units should be located so that
they are easily reachable where this is necessary
to comply with Part M of the Building Regulations.
So when it's not necessary to comply with Part M of the Building Regulations, i.e. when Part M does not apply....
And read your quote from Approved Document P (which is only guidance anyway, and not mandatory) again:
Accessibility
1.6 Wall-mounted socket-outlets, switches
and consumer units should be located so that
they are easily reachable where this is necessary
to comply with Part M of the Building Regulations.
So when it's not necessary to comply with Part M of the Building Regulations, i.e. when Part M does not apply....
