The obvious option to me is to build out the attic and create a non-conforming unit that I could move into or rent out for additional cash flow.

There no conforming lenders or banks that will allow 100% HELOC on Primary res. 

I'm not an agent, but my experience living in the area is there were quite a lot of (likely non-conforming) houses with basement units and that sort of thing.

Also, LLC's are NOT eligible for conforming residential loans which means you would have to use more expensive commercial financing.That being said, just use a spreadsheet. 

The usual loans you heard about conforming loans which are residential products that are generally owner-occupied.

However, you do have to be comfortable with the fact that you're not going to be conforming to "traditional" social norms.If you’re going to get into house hacking as a wealth-building strategy, then it’s important that the money works out in your favor.

I'm on to 30% now because I do non-conforming today and my rates are better at 30% down.  

One other thing to consider is if the space is a duplex for a LTR then there may be additional regulations to make it conforming.

Conforming conventional loans must adhere to standards set by Fannie Mae and Freddie Mac, including minimum credit scores and maximum loan amounts.Conventional fixed-rate loans with a 30-year term may be a good financing option for real estate investors buying and holding rental property for the long term, because interest rates and the monthly mortgage payment amount do not change.

The 1003 or loan application clearly states you attest that this is all the liabilities you have or know about and most loans to your LLC’s have recourse or require a PG (not all but often they do).If the borrower signs as an “individual,” on the mortgage or promissory note that is them as an individual pledging to repay, now if they sign as a Manager of XYZ LLC that is a different story (then there might be a separate PG document or perhaps its non recourse or not personally liable).Above is in reference to conventional financing and sure there are DSCR/rental loans out there or other types of products that dont care about DTI (non conforming products, hard money, private/portfolio etc).