Granny Flats can increase the value on the property by adding another dwelling (Secondary Dwelling) on the land, in essence allowing dual rent from both the primary & secondary dwelling, which in the eyes of a potential buyer, is very attractive.
For home owners who are looking for holding on to the property for long term, a Granny Flat is able to service the debt and in some cases, is able to allow the property to be positively geared. lets say you have property loan for $1M, which is not impossible in Sydney's current property market. The current rough repayments on a 90% loan at 5% interest will be around the $900 per week, the rental return from the property will average around $650 per week, this means you will be paying out $330 per week to service the loan.
Now lets add a Granny Flat to the equation.
Granny Flat construction can cost around $100,000, which you can add to the loan of the mortgage, so now the total debt is $1.1M which will also increase the mortgage repayments to $970 per week, this may seem scary at first, but we haven't added the Granny Flats rent to this yet. The rent from a Granny Flat can average out to around $450 for a property worth $1M, so that means the rental return on the property will be around $1,100 per week, while the loan repayments are $970 per week, this in essence puts $130 back into your pocket per week.
So as you can see, a Granny Flat can be both a good short/long term investment that can be beneficial to adding value to your property and help service the loan.