Using Fence as a retaining wall

We had a raised garden built along our new fence joined on one side by a deck. It's raised by about 1/2 a meter.

The way they did it was to use the fence as part of the retaining wall. However, they did add some reinforcing.

In your case, the posts may be too wide apart and you may not like the idea of using the fence for this in case of rot.

I suggest putting new posts into the ground and putting treated 4x2 along them to form the retaining wall. Put it as close to the fence as practical. Then line the dirt side of the wall with a suitable plastic (the kind that's used for building ponds). And fill up the ground to level it.

As for the area between the fence and the retaining wall, you could attach a run of 4x2 or 6x2 along the fence line to form a beam for which you can attach joists and lay some decking timber all the way along. If you raise it up about the dirt level you could use it as seating if it's wide enough.

Remember to add weep holes to the wall to allow the water out.

Alternatively, as you suggest you could use cinder blocks or even just pour concrete. Your choice may depend on price.

Having dirt touching the fence will increase the chance that the bottoms of your rails/pickets will rot. This doesn't necessarily mean your whole fence will rot; only the parts exposed to heavy moisture are at risk. Maybe you're OK with having a bit of rot on the hard-to-see part of the fence after a few years; how bad it is really depends on the wood, how wet the climate is where you are, how much sun that area gets, etc.

If you want to protect the fence, I see three options:

1. Cut the bottom end of the pickets so it's no longer touching the dirt. If needed, treat the cut to improve rot resistance (may not be needed on natural redwood cedar, but you said it was pre-treated, so any cut surfaces should be treated in the same way).

2. Add stone/gravel to provide free-draining surroundings as you suggest. (I wouldn't recommend plastic as it's likely to just trap moisture against the fence and accelerate rot.)

3. Remove some dirt near the fence (may be too much work, or against your landscape design goals).