Is the issue not that there is already legislation in place but councils are not enforcing it? Something about lack of resources to investigate? Something would have to be done in conjunction with revenue to make it more effective - if it is a stl property enforce planning and also check if a tax return has been made on the rental income.
There are difficulties catching people in the act, running prosecutions where the witnesses reside outside the jurisdiction, meeting the evidential burden required under the planning acts for a conviction.
New offences capable of being more easily detected and prosecuted are needed.
For example, an offence of having an external key box outside a house unless it is registered. Same for keypad access.