wrote:

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.

My math says 3 will do it.


First, what do you suggest to maintain overall stability?

I like these-
http://versa-lock.com/

My wall is about 3' high & 25' long- hasn't moved a hair in 5 NY
winters. It is serpentine & I think that adds to both appeal and
strength.

And how
about drainage? We get quite a bit of rain, but currently erosion is a
problem (hope terracing will help that!)

My wall started with a ditch twice the width of the wall & 8"(?) deep.
Tamp. . . tamp. . . tamp. . . lay the first course. Laid a 4"
flexible perf-pipe in a sock behind that-- and carried on to the next
course. Once the wall was done I backfilled with a bit of #2 stone
protected with landscape cloth. BTW- 1/2 of the first course is
below grade. The low side is a paver patio.


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??

That depends entirely on where you live- and who your neighbors are.
*Probably* you can do a 3' wall without a permit. [If it was me, and
I was confident of my ability to engineer the thing- I'd do a 3 foot
wall. Then another. . . ] OTOH- a call to the building dept. is
free and you might get some valuable insight.

What happens if you act without a necessary permit & get caught?
anything from make an apology and buy the permit- to tear it out and
pay a fine.

Jim