HMO Planning and HMO licensing are very different things.
Let us start with the planning side, and we will deal with what is specific to Plymouth. A typical residential property in planning terms has a class use of C3. An HMO property up to six bedrooms will have a class use of C4. Planning laws allow you to move between a C3 and C4 class use, due to permitted development rights.
However, these permitted development rights can be taken away by local councils. In September 2012, Plymouth City Council implemented an Article 4 direction taking away this permitted development right. In a nutshell, if your property was used an HMO at this time then you could continue to operate as an HMO, if the property ever reverted to a C3 use, then you lost your permitted development rights to use the property as C4 class. For more details of the Article 4 direction in Plymouth, please follow this link https://www.plymouth.gov.uk/housing/privaterentedaccommodation/housesmultipleoccupationhmo/planningpermissionandhousesmultipleoccupationhmo
After this date, if you wanted to convert a property from C3 use to C4 use, then a full planning application would be required.
HMOs with more than six bedrooms have a planning class known as Sui Generis. If you are planning to create an HMO with more than six bedrooms, then full planning will be required.
Every HMO property in Plymouth with five or more people now requires an HMO license from Plymouth City council. The HMO team as the council will inspect the property, any work that is identified to ensure compliance will need to be completed in a timescale directed by the council. If you are found not to have an HMO License, then you could face serious issues.
When buying a new HMO in Plymouth, please make sure that the property has the correct planning, and when you complete on the purchase, you apply for a new license as they are not transferable.