Your cover is based on what your home would cost to rebuild. You can check whether you have enough buildings insurance through the Building Cost Information Service (BCIS) website. It has an online tool to help you calculate the sum you should insure your building(s) for, in case your home has to be entirely rebuilt.
You need to tell your insurer if you extend your property, for example with a loft conversion or conservatory. Your belongings are not covered – these need to be covered separately with contents insurance – see Contents insurance.
As always, shop around. You may also find that you get a better deal if you buy buildings and contents insurance together. Most policies have a standard excess charge which means you agree to pay the first part of any claim, for example, the first £50 or £100. If you agree to pay a higher excess you might get a cheaper policy. Always compare what's covered by a policy, not just the price – the key policy information will help you do this. Some might be cheaper than others, but they may not offer the same level of protection.