Yes you can do HELOCS on investment properties as long as it's not a multi unit.

Here are some financing options to consider:HELOC or Cash-Out Refi on Another Property – If you or your mom have equity elsewhere, this could be a more flexible and lower-interest option.Business Line of Credit – Since the property is in an LLC, you might qualify for a business LOC, which could cover construction costs.Private Money Lender – If you’re okay with 12-15% rates, a private lender could fund this as a short-term loan, especially if the guesthouse will generate strong income.Construction Loan – Some lenders offer short-term construction financing, which converts to a longer-term mortgage once completed.Seller/Partner Financing – Bringing in an investor or structuring a joint venture with someone willing to fund the build in exchange for a share of the income.Credit Card Stacking or Unsecured Business Loans – Riskier but could work if you need to bridge a gap until the unit starts cash flowing.Would love to hear more details—especially your timeline and expected rental income.

Have any of you done this with mutli-family properties (whether 2-4 units or 5+)?

Inspect appliances for age and condition,and look over the flooring for any damage.For tenant considerations, evaluate the unit layout for functionality and storage, and check for safety features like smoke detectors and secure locks.

Maintenance can also be a factor—while newer homes may require minimal upkeep, older units might need more frequent repairs, particularly for things like plumbing or HVAC systems.Some things to pay close attention to:Zoning and Permits: Check local zoning laws and permit requirements to ensure you’re allowed to place a mobile home on your land and rent it out.

150k is steep for land acquisition for a tear down unless you are at 750k and up or multiple units.

In CA you can rent ADUs but can only rent JADUs as a separate unit if the owner lives on the property.  

One-person rental units are in the highest demand, and co-living directly appeals to this market.

It has an upstairs balcony door to the outside, and could potentially be converted into two units.

I don't get the term "DSCR loan"   I mean all loans for 5+ units are based more on the property than the buyer.