What type of use would the additional use be?
Generally Hair/ beauty salons fall under a1 retail. clinics are d1.
Change of use may be required.
Best course of action is to discuss with the planning office at the council.
Under permitted development you can change temporarily for up to 2 years from b1 to a1.
The council will determine if your use is a1 or d1.
The next issue is do you own or lease the premises? if under a lease, check it's terms. you may well need consent to change use.