building a raised bed on the outside of my garage

Is this a good idea?
I've got a horribly ugly garage wall at the back of my garden and want to build a raised bed in the garden that will sit at the bottom of the garage wall.

Obviously the raised bed will be full of soil/compost (and hopefully some plants!).
How do I ensure that this bed full of damp stuff doesn't permeate the garage wall and cause damp inside.

The garage-wall is brick built on a concrete base and pebble dashed (soon to be repainted, probably before the bed is built.)

Can anyone help me out with some advice?

 

At least an inch gap between the garage wall and raised bed for a start i suppose.

 

Build it in sleapers with a 2" gap between the sleapers and the wall.

 

There is an alternative. Why not use damp course membarne as either a liner or just at the bit that meets the garge wall.

 

Use 2 layers, the stuff is cheap. (I confess I did not notice the pebble dashed bit at first)

 

But what is safer than just an air gap?