Many side extensions come within Permitted Development rules. There are limits on the length of the extension, and extensions which exceed 3m may be subject to the Neighbour Consultation Scheme , which means if a neighbour objects the Applicant may have to submit a full Application. Two storey extensions must not cause significant loss of daylight to the main property or neighbours.
Go to https://interactive.planningportal.co.uk. Click on the type of property and you will see an interactive picture which you can explore.