For example if you have a 2 unit property and rent out 1 unit while living in the other, the expenses for the rented out unit can be written off as expense but the other unit can not as it is personal use.As to which route would be better for you to file as, Joint or Separate, I highly suggest speaking directly with your Accountant/CPA as there would be more variables to discuss about before deciding which would be the best option.

A HELOC is an open line of credit that you can use similar to a credit card, and typically has a variable interest rate.

When cap rates are broken down, it’s easy to see that they are determined by a combination of two variables: The discount rate and the growth rate of income.

The tenant is actually about to move Jan 1 so I was wondering if I could justify deducting a new door from the security deposit or if there are too many variables at play to do so.... 

Do consider how your intangible variables play in your decision making.

There are also some variables at play, you might have different concerns depending of the property and where is located, some issues you might pay extra for are radon, termites, water quality, lead.. etc. 

People who aren't point to the variable interest rates that tend to come with the territory.

Since it’s a variable rate, it doesn’t make sense.

I say this because we likely have huge inflation coming soon, and I would want to lock in that super low rate instead of a variable rate with the HELOC. 

When the bank takes the property, they’ll eventually auction it off to recoup the money they loaned you.Banks will rarely loan up to the full appraised value of the property because they want to take into account variables such a bad appraisal, damage to the property, or a drop in the housing market, etc.Banks will sometimes make exceptions with your primary residence (up to 100% LTV), because people will typically do whatever it takes to avoid being kicked out of their own home.