Do you need Planning Permission for an extension? 

House extensions does not always need planning permission. Permitted development rights provides the homeowner the authority to extend a house without applying for planning approval, provided certain conditions are met. 

All major Local Authorities, including Manchester, Oldham, Liverpool etc, must adhere to Permitted Development rights: A house extension will however require building control approval

Further details are available on:

https://www.planningportal.co.uk/info/200130/common_projects/17/extensions