I'm looking to build a featherboard fence (approx 5-6" high) connecting
the end of my house to my garage (approx 4m)... theres a couple of
things I'd like to check with some experienced people... hope you can
help.

I was thinking the best way to put up the end posts would be to
bolt/screw them directly to the house and garage wall. Are there any
issues with doing this that I am unaware of and if not what is the
recommended screw length to make them secure (standard 75mm fence
posts).

The rest of the posts are going to be concreted into the ground using
postmix. 2nd question is how deep should these go down I was thinking
about 2ft would be plenty correct?