1. If you happen to be a veteran...or a disabled veteran, have you looked into a VA loan?
2. If you are building in a rural area, have you looked into a USDA Rural Housing loan?
If neither of these options are available to you or the area you are building in, then I'd suggest that you find a good mortgage person who will assist you in exploring all of your options. I can recommend some if you contact me.
80/20 loans are available, but with 30 year fixed rates where they are now, those may be a better choice...but a lender can show you the advantages and disadvantages of various loan options.
It's your money, spend/invest/borrow it wisely.
As a wholesale mortgage lender I have access to around 35 lenders full mortgage options. The problem with the 80/15/5 is that getting the 80% first is not a problem and fits within all the current guidelines BUT I have not seen any lender getting back into offering that last 15% for the 2nd mortgage. That would mean the 2nd mortgage lender would allow a total loan to value percentage of 95% when you consider both loans. Right now I have lenders that will do a 2nd mortgage that will allow for a total of 90%, that would make an 80/10/10 an feasible option.
In the case of the 80/15/5 or even the 80/10/10, in my opinion you are currently better off looking at all of the options that you have available using only the first mortgage. If you are working with a lender that has access to all of the mortgage programs available [like me! ;) not-so-shameless plug... ] you will have multiple options and ways of structuring that loan to work best for you. Standard Mortgage Insurance (MI), Single Premium MI, Hybrid MI with reduced monthly....... the list goes on and on. There are many more options available than people even realize and the most important thing is to make sure it is structured properly for YOUR situation. Your loan should be custom tailored for you.
I hope that is helpful in some way. Contact me directly if you'd like me to run all the available options for you, I offer a full mortgage analysis as a free service for my customers.