Q: How much room should there be above the stove? In our condo, my boyfriend put a microwave where the old fan was by building a shelf. Our kitchen is tiny, so it's nice to have the extra counter space where we had the old microwave, but is it right to have it hang as low as the cabinets next to the stove?
A: How nice to have a handy boyfriend, although your letter certainly raised my eyebrows. From what material did he make the shelf? If your microwave is designed for that placement and is installed correctly, you are fine. You need to reach the microwave door and number pad to safely and comfortably remove hot plates.
The exact placement depends on the type of stove and microwave you have, the manufacturer's recommendations, and the local building codes in your area. Anything made out of wood should be at least 30 inches above the cooking surface, higher if you're using gas.
A lot of microwaves are installed properly when about level with the underside of the adjacent cabinets. This corresponds with many manufacturers' suggestions that the top of the microwave be no more than 66 inches above the floor. If you and your boyfriend are taller than average, you could get away with mounting your microwave a little higher.
Traditionally, there is 18 inches of space between counters and cabinets, though many homeowners are opting for 20 inches in new kitchens. This is great for tall coffee makers and other appliances, but in my opinion, that's still not enough space above the stove and will make your cooking space feel cramped.
If at all possible, I would find a different place for the microwave. But this is a personal choice based on what works for you and your kitchen.
I hope this is helpful. Let me know if you change anything, and send in a photo.