Your tree will eventually be more then 20 feet in diameter. This means the "drip line" will be extending over the retaining wall. This means the roots will be under the wall. OK if they are deep roots

It also depends on if the wall is a strong engineered wall with rebar and a solid footing or if something less well built.

Chris, I am not an experienced gardener yet, but can't I trim the tree every year to keep the diameter smaller than 20 feet? Or do the roots grow on regardless of the size of the canopy?
The wall is very sturdy, built to city standards with big footing, rebar, drainage, etc.

phonon71, The roots will "follow the water" If you water deep the roots will be deep. If you spray water all around every day so as to keep only the top of the soil wet the roots will spread shallow. I doubt roots would damage a to-code concrete wall. A sidewalk, yes.

Maybe if every year you prune the tree, the roots might stay small(?) But you'd have to keep at it every year for the next 40+ years without fail. But don't worry, rebar is good stuff.

My neighbor had a large avocado tree with surface roots near the fence and house for twenty years. It never damaged anything.

I have a 40+ foot tall, 30+ year old Fuerte Avocado 6' from a shallow block wall that is not even close to meeting current California Code. No problems. I had a fig and a pine tree about the same distance from the wall that have both had to be removed before they destroyed the wall. The Fig was damaging a walkway on the other side of the retaining wall. i don't think you will find many trees that will damage concrete less than a similar sized Avocado.

Don't forget that Avocados need their leaf litter to stay on the ground around the tree for best long term tree health.

My understanding is avocado has very shallow roots. You probably will find it useful to post your question on the new avocado forum. Starting April there is an Avocado forum on Gardenweb. Here is the link

http://www.gardenweb.com/discussions/2966473/avocado-forum?n=7

Just what Gardenweb needs, another highly specific, sparsely trafficked forum.

good point, kentc. avocado could be grouped with citrus, at least.

I'm doubtful that top pruning would slow down root growth.

No need to let your tree grow to 40 feet tall. Even orchards are keeping their trees shorter now so they can eliminate ladders for picking. For the home gardener who is not trying to make money from the tree, this makes a lot of sense.Pruning avocado trees for size control

About to lay concrete all around our dwarf avacado!! Is this a good idea.??It’s about 15 years old and has produced avocados in the past! Are we headed for trouble?

Terrible idea. It will cut off air and water exchange for the soil and the roots. The excavation necessary will also destroy a large fraction of the fine feeder roots.