Apportioning granny flat on floor-area basis or value basis?

Hi,

Granny flat on PPOR rented out full year in NSW.
Can apportioning of rates be done on the basis of amount written in valuer report? Or it has to be done on basis of floor-area?

I have got depreciation schedule done for granny flat only.
Contract price of whole property (purchased as house & land package in 2014) is : $495,301
Depreciation schedule for granny flat shows value $99,334

Can I apportion all rates 20% for tax purpose?

 

I don't think floor area is a good indicator. What about the land in use?

 

The example at ato website uses floor-area
Expenses you can claim

The granny flat is built on first floor (on top of detached garage) and has access from rear lane.
Tenant don't have access to any backyard.

 

@Terry_w I find value based apportioning simple to use. Hope both approaches are legal.
I have seen different tax agents using different approaches.

 

Since the GF is under the same roof that basis seems reasonable.
If it was a GF in the yard with an area of land under it then a different approach would be reasonable