There are a few different principal business codes related to real estate and rentals. You can find the full list at the link below on pages 17-18. I have posted the most likely codes here, but there are others. Also, these codes are only for statistical purposes, so if you don't find a good match you can use 999999.
Real Estate Property Managers - 531310
Other Activities Related to Real Estate - 531390
Lessors of Real Estate (including mini-warehouses and self storage units) - 531100