this has disaster written all over it.

do you have any experience in home building? i assume not if you even think this is somewhat viable idea given your economic situation. you have like 20k in the bank and you want to spend other people's money (probably your parents retirement nestegg) on building a new duplex infill because you feel like you deserve to own a property even though you havent really done anything to earn it?

with going rates, even with the economic downturn (CAD/USD exchange is keeping material costs high), residential construction is about $150 - $175/sqft for basic finishing. that is not factoring in inevitable fuckups, permits, demolition, adding a second service to the lot, garage, dumpster rental, = etc..

i dont know your personal situation, but i feel like at 40 you're a bit too old to be going to the bank of mom and dad for this kind of thing. you've wanted to own a house for a long time and you only have $17-20k in the bank? well son, you done fucked up either with spending or income and unfortunately you missed the boat. right now your best opportunity at home ownership might be through inheritance. be an adult and dont take advantage of your parents with some pie-in-sky idea you havent even sat down and done the math on. you WILL get taken for a ride by contractors. you WILL have months of delays in the permitting process. you WILL feel badly for wasting their money and delaying their retirement.

here is my very sincere advice:

sit down and write a business plan and figure out exactly how long it will take you to break even in a worst-case scenario (major construction fuckups, vacancies, etc..) take your plan to a real lender and see what their response is.