ADU, Manufactured, or PreFab?

[Jacob I Strauss]

As simple as It sounds which one do you feel is best and why?

I'm asking to see what makes most sense to pursue given time, money, and what scales best with economies of scale?

Quick back story:

I am looking to buy a home that has a decent size lot that is zoned R4 for about 485,000. I noticed they made an unpermitted addition to the home that added about 500SQFT to the home buuuuut it also has its own entrance. So it got me thinking after I demo it for the city and make corrections to the framing etc etc... that I could wall off that part of the house add 1 bed. Then like magic I have a Duplex with 700SQFT for each side 2BD 1 BA.

The lot itself is also big enough to put an ADU in the back and I have a contractor that says they will charge APX 120 per SQFT and pricing gets better with size so it really got me thinking this home for 500,000 isn't really a good deal though adding a 1200 SQFT Duplex in the back does allow me the ability to house hack it as well while cash flowing about 3,000 pure after everything is all said and done.

My numbers are like this:

Purchase: 485,000

lender fees: 21,500

Initial stabilazation: 120,000 gutting job on 1,000 SQFT of permitted and 700SQFT unpermitted space. Needs new everything roof, electrical is litterall glass fuse, and some foundation work.

Holding costs: 15,000 projecting 3-4 months

ADU: 200,000

Total: 821,500

ARV: 900,000-1.1M

Cash Flow: 2,944

COC return 9.4% but thats with me in one of the units losing 2,000 in rents.

prefabs are cheap and fast but still good quality. Nit sure if they could command the same rents as a srick or manufactured.

Manufactured have about all the same benefits of traditional stick built but are generally faster to build and slightly cheaper.

Stick built- most expensive but made from common materials, easier to appraise, but take longer to construct. 

So what does the community think ATM in San Diego the days of SFH are dead and ADUs provide a great vehicle to get more out of your property but which one is best for which situations?

As always thanks a lot!!!