Hi.  This is a tough task.  What you have going for you is an engineering degree, which is always respected, and work experience related to buildings.

As for preparation of the portfolio, simply check in to your local community college or nearby satellite campus of your 4 year university to take art and design classes as a non-matriculated student.  I was always fairly capable in drawing as a kid, but I wanted the extra guidance from a teacher.  Also, do you engage in other crafts, such as photography, sculpture, or other type of handiwork that shows creativity and 3-dimensional thinking?  You should include a variety of things like that in your portfolio.  My portfolio was judged to be very competent by architects I knew, but I don't think it was splashy or artsy enough.  This was before all this software was the order of the day.  I had bought those opaque plastic binders with about 10 to 20 sleeves and just inserted measured tight graphics (perspectives), freehand drawings, and a few examples of hand drafting from community college.  Maybe some committees thought I was UPtight.  Well, I can be.  It's a lot like advertising.  A certain baseline of slickness is expected, especially now.  Sheez, I saw one portfolio of an admitted UVa student with beautiful work, including an ink rendering of the skyline of an American city.  I knew I wouldn't be getting into UVa. LOL.

Never fear your undergraduate degree.  The best all-around performers in my particular class were from engineering and commerce, more so than fine arts.  Also, go for a program that uses 3 years, or uses 3 years and a summer term.  Avoid the 7 or 8 semester programs.  That's just nuts, especially since the average age upon entry is in fact about 27 or 28.  Good luck.

thank you very much.