Firstly, the contractor who is building your conservatory should offer an independent insurance backed guarantee of the work, covering the possibility of the firm ceasing trading and covering the costs of any further work to be done in case of mistakes.
In an ideal world, you should notify your insurance company before any work is carried out – during the build, the conservatory will be stored on site, and could be damaged or stolen, including the glass panels. Check if your policy will cover this eventuality.
When the new conservatory is finished, it becomes part of the structure of the house, and it should be covered under your existing buildings policy, but you must inform your insurer of the extension and raise the sum insured to cover the cost of rebuilding.
If you have a separate contents policy, ensure that the items kept in the conservatory are included in the policy.
Conservatories may need more maintenance than houses – insurance should cover sudden damage, such as storms or a roof tile falling on the glass roof, but will probably not cover general maintenance such as a leaky roof where water is coming in (although you may be covered for water damage to the building as a result).
The purpose of insurance is to get you back to the position you were in before the incident occurred. Make sure you are insured to cover costs, read the wording of the policy or get an expert to talk you through it – the only way to be sure as to your covers and exclusions is to check the conditions of the policy – make sure you understand what exactly is covered.