We have found that getting rid of the tv and hanging out with my kids in the living room has brought WAY more happiness than buying extra doodads. 

It's tight but we're working on moving out right now and it's awesome being able to count on some extra income when you move on.

So if 75% of your rental income is greater than your mortgage then you can count the extra as income when applying for your new loan.

Buyers can justify the extra principle payment because in theory they will own a more expensive house at the end of the mortgage.

@Christopher Hall, @Seth Williams mentions two things that are worth extra comments.

Not a fun topic to think about, but if you want to keep the property and they want to sell, it's best to know now, before you go into a partnership with them.If you go this alone, be prepared to pay 100% of everything - that way any money your mother gives you is extra, not something you're counting on to pay the bills.It is admirable that you want to help your mother, but make sure you can afford to do so without jeopardizing your own finances.

What I get caught up if you put the minimum down on FHA and have the extra cost of Mortgage Insurance is it worth it?  

Goodbye, mortgage interest deduction, hello extra cashflow.And THAT's when I finally cracked.

The extra two units may make it a steal.

If the roof is really high most will charge extra.