Yes, an attic conversion can affect your house insurance, and you should tell your insurer about it. A conversion is a notifiable alteration to your home, which means it is a change your insurer expects to be informed of. The most common practical effect is on your rebuild cost, or sum insured.
The reason is simple. Adding a finished, habitable room increases the amount it would cost to rebuild your home if the worst happened. If your policy still reflects the house as it was before the conversion, you could find yourself underinsured, which can reduce a payout at exactly the moment you need it most. Letting your insurer know allows them to adjust the sum insured so it matches your home as it now stands.
Notifying your insurer is also part of keeping your cover valid. Failing to disclose a significant alteration can give an insurer grounds to question a claim later, so a short conversation or email when the work is done is time well spent. This is separate from planning and Building Regulations, but it belongs on the same checklist of things to tidy up once your conversion is complete.
Having your engineer’s or architect’s Certificate of Compliance to hand can help here too, as it shows the work was carried out to the proper standards. We build every conversion, whether an attic bedroom or a dormer, to those standards as part of our process. If you would like to talk through a project from start to finish, arrange a free assessment.