@Jo993 HMO and STL are two different things.
A house of multiple occupancy is where several unrelated people live in the same property, sharing some facilities. There is a licence required for this from your council, after meeting various criteria and being inspected.
Short term lets are normally used by folk on holiday or travelling for work. A different licence is requred for these.
Check on your council's website for full details. Planning consent is also required for HMOs, and may be for new STLs.