Part of the plaster on ceiling is rotten

Hi, maybe not, see it maybe cheaper to fit a new light that is larger then the area that is damaged, with the base of said light covering the area, failing that it maybe that a small section can be cut away then re-boarded and patched in, this is almost impossible to see when do the right way. DN Maintenance

Possibly - to get the best finish, though a patching job might suffice but may be false economy if you need a plaster anyway - depends on the size & condition of the rest probably.

If the damage on your ceiling is little you can do a patch up job and make good. but if the Damage is more then just a little bit it would be better to re skim the ceiling

Hi. Not sure what you mean by 'rotten'. Have you had a water leak/ ingress into the existing fitting? If you do decide to replace the fitting thn mybe re-post the question to the electricians here, the type of light fitting in a bathroom and its proximity has critical aspects

Obviously this needs to be sorted out as it's an electrical item in a bathroom. as most bathrooms ceilings are fairly small I'd suggest the easiest option is a re-plaster, the time spent doing a repair is easily comparable to just getting it re-done and doesn't run the risk of the repair showing. The question I'd be asking is why has it rotten is it due to high condensation levels and is the ventilation adequate?