Have you considered the advantages of hiring temporary office accommodation as opposed to the cost of building.
As a business owner the GST on hire of the building can be claimed, plus the full cost of the hire is a tax deductible expense in most cases. Now generally there is no depreciation allowance when you own a building.
One client recently had a price to add a small office with a porch to the side of their factory. The cost was in excess of $40,000 + GST. They hired a middle size veranda cabin off us and found it a far more financially viable option for their medium term office space problem.