They are called "accessory dwelling units" and are only allowed in properly zoned areas.

But I am in no hurry to give up the tax benefits of buy-and-hold real estate: The ability to write off the interest, insurance, property taxes, 1/27.5 of the property, 1/5 of the appliances and accessories, and 1/15 of the landscaping, every year.

I have a different method than Michael Paris on this one, I give out an rfq to the lumber yard, I have the architect call out types of wood, bolts, and other accessories (osb, doug, etc) to obtain apples to apples bids; on my rfq I put any miscalculations in quantity is going to come from their end. price lock for 2 months.

Some communities up north allow "Accessory Dwellings" on SFR if you want to do some house hacking.  

Additionally, the reason we hesitate to sell is because we've got an accessory apt in basement with income stream as well as a separate office for where I work every day (self employed).

I think it's still the case here in Charleston County that you can add an accessory dwelling of up to 800 sqft as long as your lot meets certain minimum lot size requirements.  

she is giving house back because she really can't live on her own any more and she is going to move in with my cousinyes i am sure the furnace is dead we been dumping money into it over the years like 4 years ago the guy told me it was so inefficient that its not worth fixing but it is definately deadthe problem with putting the old one back in is the whole setup is being changed up because will be putting in a furnace and a water heater separate along with some acessories i would leave all the acessories and new piping and just remove the water heater and the furnace. i was thinking about filming all this with a newspaper for the date and the old one before it is taken out.don't know if i should call the bank and ask them because if we don't do anythign and leave it they going to have to upgrade it anyways so they would be saving money on the piping and the accessories if i take just the units backthank you for your time

Note that they only approve 28 new units a year:http://building.arlingtonva.us/project/accessory-d...

Or if it's really cement like, you could do an exposed brick look and really up your rent (go with ultra modern accessories for that loft look).  

As far as I know, Accessory Dwellings can not be sold.