Think of the costs of moving - stamp duty, legal fees, furniture removals, surveys, searches - it can add up to a small fortune.
Then you will want to change the carpets and curtains, and redecorate, possibly a new bathroom and kitchen too.
If you are happy with the location of your house, get on with your neighbours and generally like the house - stay put and extend.
Interests rates are likely to go up - I don't know for sure but I expect them to go up by at least 2% over the next year or so. Be careful about borrowing too much money.