Either way way, it detracts from the aesthetics of your space. 

Depending on aesthetics, amenities, & location, I would run numbers for $3,000-$3,200 there. 

If the neighborhood could accommodate a 5th unit and it truly adds to the property for aesthetics as well as utility (i.e. is there a place to park, nearest city utility access, septic system capacity, etc) it would certainly warrant investigating the potential the ROI for building a unit.  

So you would be at a total of just under $300k to maybe $350k (to be conservative) Where in Pasadena, assuming a great flow and design that is cohesive and aesthetically pleasing would give you a value of about $550 to $700 per sq ft : at $600 per sq ft value X 1000 square feet: that would be an added value of $600k (depending on neighborhood and comparable homes )Spend $350k to get a value of almost double: $600k Take timing into account, holding costs, utilities and think if you will want to live in the property ?

So, if you planned to own it for 30 years you could be relatively sure that you will replace the:  plumbing, windows, siding, floor joists/piers, and roof (again) . . . that's not touching aesthetic/cosmetic things that will possibly be replaced along the way. 

However the aesthetics of modular houses are more modern (sell easier) and the turn around time reduces carrying costs substantially.

But I've never been great with aesthetics. 

No major aesthetic problems in the community, some homes are well kept some aren't.  

We were planning on making renovations to the bathrooms and kitchens plus obvious aesthetic work but upon seeing the other three units I fear there are some more serious issues and wanted to get some feedback from experienced REI.

I've had the displeasure of starting new associations as well as serving on the board of multiple hoa's, and 90% of the time it's an absolute nightmare for various reasons.That being said most of the time an HOA is a necessary evil to protect property values and aesthetics.