When you extend into your loft, you’ll almost certainly need extra lighting and power points.
This isn’t something you can do yourself, as any electrical work carried out must comply with Part P of the building regulations.
The easiest way to make sure it does is to use an electrician registered with an electrical competent person scheme. (Find one here.)
This means they’re able to self-certify that the work meets regulations and will notify building control for you if needed.
Your electrician will be able to discuss the best lighting options for you and can install your integrated smoke detectors.
Depending on the condition and age of the existing wiring in your home, you may need to replace some circuits or need a new consumer unit, so bear this in mind, as it could be an expense you haven’t planned for. Your electrician will check this before wiring your loft rooms.
When the work's completed you'll need to make sure you receive a building regulations compliance certificate and an electrical installation certificate to show the work carried out meets the BS 7671 UK Wiring Regulations. (Chase your electrician for this if you haven't received it - you'll need it if you decide to sell your house later.)