New construction multifamily 12-15 units considerations

[Sabi Const]

Hi everyone, I would like to ask your opinion about constructing multifamily buildings. 

I have been presented with an opportunity in a town where builders are buying lots or properties and demolishing them and putting up 12-15 unit rental apartments. Then renting them for approximately $1,700 (2 bd/ 1bth) to $2,100 (3bd / 1bth). The total cost of the construction project is approximately $1,000,000. 

A few questions / thoughts... I was told that as long as the land is in contract to be purchased, the construction loan could be approved by showing renderings to local banks. I have been given requirements from 20-30% down payment and the rest financed through completion. 

If my numbers are correct, at the low end of 1700/month x 15 units x 12 months = $306k

Discounting 5% for vacancy, 10% for management company, 25% for expenses and debt servicing PITI, it seems that the return is approximately 18% per year.

Please chime in and let me know what I am not taking into consideration beyond what I mentioned above and what your thoughts might be on this type of investment 'opportunity'. 

Thanks!