I own a restaurant, and since COVID-19 it was severely affected. Can I change my restaurant to an office? And do I need planning permission?

You will not need any planning permission to change it, as it’s in the same planning class (Class E) and you do not need permission to change use within the same class. From Sept. 1, 2020, the government designated a new planning class, called Class E.

Class E includes:
E(a) Display or retail sale of goods, other than hot food
E(b) Sale of food and drink for consumption (mostly) on the premises
E(c) Provision of:
E(c)(i) Financial services                    

E(c)(ii) Professional services (other than health or medical services)
E(c)(iii) Other appropriate services in a commercial, business or service locality
E(d) Indoor sport, recreation or fitness (not involving motorised vehicles or firearms)
E(e) Provision of medical or health services (except the use of premises attached to the residence of the consultant or practitioner)
E(f) Creche, day nursery or day centre (not including a residential use)
E(g) Uses that can be carried out in a residential area without detriment to its amenity:
E(g)(i) Offices to carry out any operational or administrative functions
E(g)(ii) Research and development of products or processes
E(g)(iii) Industrial processes

In short, any shop can be changed without planning permission or applying for a change of use.
If, for example, you have a shoe shop, from a planning perspective, you can just go ahead and sell baked goods as both fall within this new Class E. Similarly, you can change it to an office or gym at your leisure, without making any planning application. However, some of these changes in use, for example to sell food, will need a licence from the council, in the case of food, for hygiene reasons. So please do check all the relevant regulations, even if you don’t need planning permission. If you want greater peace of mind you can put in a Certificate of Lawfulness application to establish that the change of use falls within the Permitted Development rules.