What is relatively new is the “mortgage stress test” where banks shall add an additional 2% on the top of the landing rate to make sure people can truly afford it.

In addition to what's been said above, the invoice should clearly state the address where the work was done and your company's name.

We were at a loss for how to get cash to purchase additional property, and are considering taking out a mortgage on the duplex.

Sometimes it's related to their current living arrangements and wanting to minimize double payments. 

Neither my definition or your is perfect, and I by no means minimize your success.   

@Nadir M.Two additional thoughts:1) A central boiler is a cost trap for owners.

In addition you'll have to make the monthly interest only payments.

There is additional reserve requirements when buying more than 1 unit, so get up to speed on those so you are prepared to have those in place at the time of closing your loan. 

These are professionals with additional training and a stricter code of ethics.

I have my own construction company in addition to my real estate business so my situation is different than most as I have the entire scope of work completed by my own crews aside from a few items like plumbing that we sub out.Before that, I acted as the GC as owner builder and hired each individual sub.