Build in external laundry

I own a 4 bedroom house in a large town in regional QLD. The laundry is outside under a verandah. I believe it can be built in without need for any major structural work (uprights, roof, concrete floor etc all in place). The room will be 4.8m x 2.8m (13.4m2) and will allow some much needed storage space as well as the normal benefit of the internal laundry.

Not sure how much to spend, as want to make sure the value added is more than spent. Is there a rule of thumb to apply to the value of such an addition?