You have given the method, what about demisions & design? for example who has carried out the calculations for load bearings, point loads, super imposed loadings etc.. Soil conditions..
If you don`t have P.I. and require a mortgage the bank will not grant you approval or allow you to draw down the stage payment.
I deal with banks in this regard and they are very strict, they need to protect their money.
Look its fine if you want to build your house with your own fair hands, but please go about it the correct way. If you dont you might pay dearly for a very simple mistake.