How much time / cash / other are each individual expected to contribute?

A great place to both learn and contribute back to those just starting out.....

Start with BiggerPockets Ultimate Beginners Guide (free).

Everything else; relationships, making connection, closing deals, and ultimately making money, is the result of concerted efforts.

Here are a few strategies to consider for funding a down payment:Partnering: Like Gregory mentioned, finding a partner who can contribute to the down payment in exchange for equity or a profit share is a solid option.HELOC or Cash-Out Refi: If you have equity in your home or another property, a home equity line of credit (HELOC) or cash-out refinance can provide the funds you need.Private Money: Reach out to friends, family, or private investors who may be interested in lending you the down payment with a promissory note.Seller Second: In some cases, the seller might agree to finance a portion of the down payment as a second mortgage.Live Frugally and Save: It takes time, but reducing expenses and setting aside funds from your income is a tried-and-true method.The right approach depends on your situation and deal structure, but a mix of creativity and resourcefulness can go a long way.

I think the ultimate situation for what I described above (not the plan, but a possible outcome) could look like this:Rent main house by the rooms while constructing ADU.

Ultimately anyone else's conversion rate with a given type of marketing in a given market does not necessarily mean you will get the same rate.

Ultimately, clear communication and expectations handled in a respectful manner can avert any issue.

I envision this portfolio supporting me during my retirement years—though I still plan to work, just in a different capacity than I do today.I recently finished reading "How to Invest in Real Estate: The Ultimate Beginner's Guide" by Josh Dorkin and Brandon Turner, which has been incredibly helpful.

What your deal would ultimately entail is a buyer who trusts you enough to give you $125k and trust you to pay it back while his property is encumbered by both your seller finance note and your original mortgage for $125k.