The contractors were saying transport and disposal of the concrete was the issue. 

It is currently rented 3 of the 5 units (they renovated the other two). 1block from public transportation.

The apartment is located within a 15 minute public transport distance to the old city center and also 15 minutes to the beach in Valencia. 

Here are a few factors you can consider to help you identify desirable areas:-Proximity to the base: Look for areas that offer a reasonable commute to the base, considering distance and transportation options.

The ROI in my experience is always higher in suburbs as long as they're close enough to public transportation and the city.

Little difference in the lot prep costs but what you might save in efficiency in the building process is often eaten up by transportation of the prebuilt structure to your lot.

Even people with a house full of furniture may want the decorations and some of the furniture.What you are offering to a potential buyer who is not an investor is a way for them to sell their furniture and avoid paying high transportation costs.  

You could also structure it like a mini (unregulated)syndicate and have a preferred return and a waterfall.

Generally in the Pittsburgh rental market the trend is towards walkable areas with good access to public transportation.

Walkability, public transport, etc are other things that should be considered.