We have a slope in our back garden that we want to terrace. The rise
is a little over 8 ft, and the run is 20 feet. We're thinking 4
terraces, each about 5 feet wide, with walls about 3 ft to 3 ft 6.

First, what do you suggest to maintain overall stability? And how
about drainage? We get quite a bit of rain, but currently erosion is a
problem (hope terracing will help that!)

Second, do we need building permits to change the landscaping in our
own back yard? And if we don't get permits, what might happen??

Thanks! :-)