I'm looking for an architect and builder who specializes in condo conversions in Denver, Colorado. 

I went through Fortune Builders Mastery class about 6 years ago and also dove into real estate full time at the same time.Since then, my wife has joined me so we both do this full time. 

That's one reason I appreciate Fortune Builders - most of their students approach this like a job, and when they send you a proposal, it's DETAILED! 

I know some custom builders doing foundation-up new builds with two car garages and 600-750sf 2br units above.

Realtor told me a builder offered 139k, which I highly doubt because she also said the builder doesn’t know if he can even build on the 2.5 acres yet so doesn’t make sense for him to offer if he doesn’t know...but just my opinion.

Fortune builders charges absurd money to teach this stuff.

I've also been touring homes of new construction subdivisions, speaking with builders to gain some additional insight.

@Mel Hayes standard Landlord policies are not 6 months, but Builders Risk policies are. 

What are some companies who could build a 4-Plex in Northern Virginia? Suppose I have a half acre or more. The company doesn't need to be local, but it needs to meet local code of course.

Long term buy hold investing  ( what the vast majority do and want to do) then you have transactional real estate that those that work in the business doIE RE brokers   flippers  builders  court house steps buyers etc etc.