Dividing IP 3br Split level house to 2units

Hi
Has anyone done a conversion of a house to two units? We want our IP to make us more cash flow long term , so r debating either
- build from scratch a gf in backyard
- or converting the downstairs sg LUG plus bedroom plus undeveloped under house into a unit. The house is on a slope so top floor is level with backyard and at front can be accessed via steps.

Or- can we do both and have three income streams?

There's space in front yard to put car parking/ carport to replace garage.

Would love to put the gf in front yard but I think the building set back line prevents that. Who do we ask - council? Can this be modified?

Any thoughts on costs or approach, would be welcome. We would be paying for all trades bar painting.

Issues:
- No side driveway, the only side access is a footpath. To build a gf , access by machines might be tough.
- slope of land.
- heights are good. Garage slab would need replacement it's all cracked etc.

Thanks.