If you already have an in-law apartment or if you are thinking about building one, you should think about renting it out to tenants. If you already have the in-law apartment built in your backyard and your side yard, renting out is pretty simple. All you have to do is decide how much money you would like to make per month and then price your apartment there. You should then check similar apartments in your area to see how much people are paying. You should be able to find the usual rent rates for granny flats in Victoria, Australia with a quick Internet search. However, if the apartment has not yet been built, you have a few more considerations before building.
If you are thinking about building an apartment in your house, you should be able to find a company that will build one for you. Typically, in-law apartments are three bedroom structures. They are usually a bedroom, a kitchen, and a bathroom that sit beside or behind the main house. Many companies in Australia will build these apartments. You should look through a bunch of different companies to decide who you would like to work with. Typically, these companies will give you an estimate that quotes how much they think it will cost to build the apartment as well as how long it will take.
This is probably going to be your biggest expense and the one you should take into account first.
Plumbing for your in-law apartment is going to be simpler and less expensive than the plumbing expenses for your main house. This is usually because the infrastructure for plumbing is already set up. You do not need to run all new lines from the main sewer drain or anything like that. A plumber can simply add on to the system so that it runs to the new apartment in the backyard or side yard. This is going to be much less expensive, but it obviously will not be free. This is another expense you need to take into account in addition to the price of construction.
In addition to plumbing, electrical is going to be another major construction expense. Just like the plumbing, it should not be as expensive as wiring an entirely new house. This is because the infrastructure for the electrical system already exists. Also, the in-law apartment is much smaller so it should require much less wiring. This is another expense that you should have estimated.
Once you have the expense of construction figure out, you should know how much it will cost you in total to build an in-law apartment. Once you have that figured out, you should then figure out how long before you would like that to be paid off. How much time you are willing to wait before making your money back will determine how much you should charge for rent. Once you have your rent figured out, you can then decide if that is an amount of money people in your area would be willing to pay.