When I got my attic converted, I phoned my insurer Allianz and asked what to do, as officially my attic conversion is not a room, it is just fully floored, wired, plumbed, heated and insulated storage area with velux windows that has a sofa, pool table, dart board, sound system, TV, WiFi etc.
Allianz they said they considered it a room and amended my policy to a 5 bed rather than 4.
Allianz also said having an attic conversion in no way affects my home insurance policy, all benefits and cover remain the same.
I asked them to email that to me which they did and keep this email in my home insurance folder.
That has been my experience, if in doubt phone your insurer.
Although the biggest room in the house floor area wise anyway, my attic conversion was never going to qualify as a room, so the builder suggested a slightly narrower staircase which meant less had to come off the bedroom that was being reduced in size to accommodate the attic stairs.
This single bedroom is now 3.1M X 2.2M or 6.8 M2