Do I need building regulations on a property that is 20 years old?
Your question is incomplete. If you mean do you need to be in compliance with current building regulations, the answer is maybe. If you have a private home, and you are selling it, the answer is no. You are grandfathered in that case to the regulations that were in place 20 years ago. If you are going to renovate the property, then the answer is yes, you need to upgrade to current code. (Such as fault protector outlets in the bathroom, or railing requirements when replacing a deck).