If you want to house farm then surely mushroom offices are by far the best?
Clear twice a day (say 8am and 8pm) and you net (without bonuses) $400 and 40xp for an outlay of 400 parts.
Retro houses, (a good time guide if KEM farming) clear 4 hourly (8am, 12pm, 4pm, 8pm) net $240 and 24xp for a cost of 380.
To better the mushrooms you need to clear retro houses 7 times per day (8am, 10am, 12pm, 2pm, 4pm, 6pm, 8pm) and is that really feasible logging in that often?
They may take up more space but I reckon if you want to farm the best return, mushroom offices are the best. Just build how ever many retros you think will look good and then mushrooms all the way!
