An ex development sales office or the like are not overly expensive and a small composting toilet would negate a dedicated black water septic. A greywater holding tank with a pump may be acceptible to tbe Council.
When it is no longer needed you can sell it for close to what you paid, or keep it as a workshop.
We are working on a housing development site where the sales office was up for sale. 10m x 4.5 m.
All they wanted was $5 000 and the removal would have been around $3 000. It had 2 a/c units, insulated, security screens and wiring.
You could also look up old bus style RV's. I have seen them from $20 000 up to squillions but fully self contained.
If the new building needed a treatmant plant or septic, you may be able to site that so it can swing over to the house later.