What would create the fastest return on investment for building apartments?

I want to know if I should do a union carpentry or plumbing apprenticeship. Once I complete one of these trade focused areas, what would create the fastest return on investment for building apartments to rent out? I do all the plumbing myself and hire out everything else like say the framers or purchase the materials and do all the carpentry myself while hiring out the plumbers, hvac, electricians? What would pay off the fastest? I could create a side social enterprise that builds fully green apartments. Is there a way I could get free building materials? Like lumber if it's a social enterprise? I could run an all round handyman general contractor that does all the trades for residential and light commercial contracting. My business would work with my social enterprise to build apartments along with regular contracting as well. The rental profits go to building more apartments, fund conservation projects like conservation easements on farms and forests, worker owned aquaponic greenhouses and grocery stores where the lowest paid worker gets a living wage because it is employee owned along with many other good investments and donations.

I appreciate your enthusiasm for creating a major social good, but I think you may be putting the cart before the horse. All union construction trades make good money when working for a contractor; you would have to investigate which does best in your area to maximize it. I would just go with the one you feel most interest in, as there are few things worse than being committed to a job you hate.

As far as building apartments in a self-sustaining community improvement scheme, that requires much more than skill in one trade to pull off. You would need skill and aptitude in financing, construction management, wrangling bureaucracies and community groups, and likely much more. Construction materials will not be free even to a noble enterprise unless you find a wealthy benefactor. You can find some things with creative scrounging, but not a steady stream of all you need to build apartment buildings.

I would start by surveying your area for opportunities and organizations that do similar things and asking for local advice. What sort of resources and character does your area have now?

To add to what Glenn has suggested, a lot of capital would be needed and/or business partners.

The whole issue of having apartment buildings infilling areas that used to be restricted to single family homes is *huge* in British Columbia, Canada, right now. We've got an ongoing housing shortage, particularly for lower income people, that current building and planning processes haven't made a dent in. However, the push-back is huge, so I suggest Wesley that you look at the codes in areas you were thinking of building and determine if local Counsils are willing to consider or support your goals.

The only ones I know of that have consistently met the need the OP is identifying, are run as Co-operatives. I have heard of a number of long-term, stable co-ops in the lower area of the Province (I simply don't have data on elsewhere). Unfortunately, they're seen as weird and atypical, rather than being embraced as the way of the future - probably because everything's "about the economy" and co-ops are more about quality of life and community building! We have gone through at least a 60 year period where "mobility" and "independence" and "autonomy" are the ideals that have been pushed, and modelled by the Entertainment Industry among others, with very few genuine examples of cooperative teamwork or community building. It will take more than just a new model of housing to fix that, although better models of housing will certainly help!

Speaking only to the profitability of each individual trade - I have spent most of my career in the construction industries framing, roofing, remodeling, and lumberyards - don't underestimate the long term damage to your body. The carpentry side of things will degrade your body relatively quickly. I can't speak to the electric or plumbing but I would imagine it is slightly less extreme. Short term profitability doesn't take into account your back surgeries and joint replacements later on.

In the 1950's in Australia there was a shortage of materials, so practical sized homes were built.
About 120 sq. M which were very adequate and still are today.
Whatever you build;
-  vary the design a bit
- have 2.7M ceilings minimum
- try 'sweat ' equity where the home owners help build.
-  design a garden shed into the house
- build cozy sized not enormous sized rooms.

Step one is to find a Union that is willing to accept you as an apprentice.  It is not as easy as you may think.  You don't simply put in an application and forget it.  Try to find someone in the Union already who can sponsor you.  Look around for non union jobs that have an opening for an apprentice in any of the fields you are interested in.  While living here my son worked as an HVAC apprentice AFTER taking classes at our Jr college for it.  He also did sheet metal and a bit of framing. Basically while they were working on new buildings he was like where do you need me and who is willing to teach me.  

For sure don't skip over the fact that all these careers are physically hard on your body.  Shoulders get damaged by hauling around heavy tool boxes (DH has had 3 shoulder surgeries), all your body gets mashed into positions that are uncomfortable and  bent in ways our bodies object to.   DH is a retired HVAC man.  All but one of his neck vertebrae have been fused due to damage, a section of his lower spine has been fused,  His hands have some damage, He slipped off an air handler and took bad tumble at work that required him to be removed from a small triangular space by the EMTS/fire rescue.   That one gave him a bad concussion and some permanent brain damage  which lead to his retirement.

Does the union movement manage apprenticeships?