Not sure how you are with building, but around here you can get
a 24x36 2 story garage kit for about $6000, if you are able to do the wiring plumbing and cement work also, you could probably finish it
for less than $15-18 K, if you have room for one on the property

for about $7500 you can get a 30x40 from Builders Discount Center

thats about 1200 square foot, just on the top floor, and you can frame off about 15-20 ft on the bottom level and still have a garage

if you shop around you could probably finish it off with high end materials for between $20-25 k

that way you'd have your space and privacy, but still be right there

just a thought