There’s already a detached garage built in the back of the property too, so the is shouldn’t be much of an issue.In terms of comps, there are not any comps with a garage conversion, only with additional bed/bath.

There should be decent cash flow and appreciation and it being directly next to my other property makes it much easier to manage.My question is: How, if possible, can I predict what kind of added value there is in having a double lot? 

@stephen @Stephen Stokes I'm going to live in the 3 bed and rent the 2 bed as a monthly rental or STR

Your work is double in that you need to get a firm grasp on market value and a good idea of rental returns, vacancy, etc for your chosen market.

Almost double...

If I want to evaluate quadplex consisting of 8 bedroom, 4 bath should I find a similar comp for 2 bed 1 bath in the neighborhood and multiply that by 4? 

Houses that look very similar on paper (same neighborhood, style, square footage, year built, beds and baths) can have very different market values depending on things like interior light, flow, ceiling height, privacy, yard space, neighbors, curb appeal, master bedroom size, kitchen counter and cupboard space, parking options, driveway slope, proximity to noise or businesses, parks, schools, etc. - to name just a few things. 

A 2 bed 1 bath upstairs and 2 bed 1 bath downstairs 1300sqft bungalo on a 80×160 lot.

Beds and couches are the most expensive, and I’d get a new sleeper couch to allow for more guests.

The administration is also looking to double the size of the IRS and increase audit frequency.