In considering this question for a property in BC, it would be easy to read the relevant provisions (in Division 14) of the Local Government Act and conclude that what is prescribed by those provisions would be allowed, but not more. However, on careful consideration you should not stop there. The common law may apply to expand your rights (although this does not jump out from reading Division 14).
A trilogy of cases has come out of Ontario that speaks to how the common law creates grandfathering rights that are not expressly found under the Ontario Planning Act. The three decisions of Re TDL, Brougham v. South Frontenac , and Fraser v. Rideau Lakes affirm and provide rights to property owners that allow them to expand or evolve the use of their grandfathered property.
Although the provisions of the Ontario Planning Act are different than the Local Government Act, it is not fully clear that the rights stated in the trilogy of decisions would not apply in BC. There are arguments to be made that the common law rights are not ousted by the legislation. Therefore, property owners may have greater grandfathering rights than initially appears and may be able to evolve the use of their property.
If this situation arises for you, you can expect that the local government will point to Division 14 and say “that’s it”. But if there is money at stake for your proposed use, or you really want to demolish and rebuild the family cottage, consider looking further.