Many side extensions come within Permitted Development rules. There are limits on width, which have to be less than 50% of the width of the original property. The extension has to be subservient to the main building, including being set back from the main building and with a lower roof. There also has to be a 1m gap between the side extension and the property boundary.
Go to https://interactive.planningportal.co.uk. Click on the type of property and you will see an interactive picture which you can explore.