If it's not possible to train your wife to not open the overhead door when you are in there working, then the out building is better. A window hung air conditioner will cool you in the summer but you may need heat in the winter. If you are out in the country, then the bushes behind the out building will be your bathroom . If the out building has an overhead garage type door, that's a plus. It's nice to be able to roll out an assembled airplane in one piece for testing or just to check how it's looking during the build. 16' x 28' is good size yet easy to heat or cool. Is it insulated? Costs are going up here, heat, A/C, insulation, lighting, electricity, and don't forget a good shop radio or TV. Of course if your wife took offence to you're request to not open the garage door, you may need a bed out there too .