[Wavytone]

Yes, I built one a long time ago in Canberra.

The block we had was heavily treed and the only way to get a decent view of the sky was to go up, and like your idea I built a wooden platform 3m x 3m with a handrail at the level of the eaves. It was fixed to the house (double brick) on one side and had two 100mm sq. posts on the other side, with wooden decking to walk on. It was fixed to the wall at a corner of the house so there were two double brick walls meeting at right angles, running along two sides of the platform; this is necessary to ensure you don't stress the wall and cause it to fail. Alternatively if there is an interior wall at right angles this would do to stabilise it.

I had an 8" f/7 Newtonian, with a heavy equatorial mount, supporting this I had a 100mm dia. pipe stuck in the ground coming up through the platform, something like 3 metres tall. With all this length in the air there is no point burying it deeply, mine merely had a concrete paver underneath it. The pipe was filled with sand, which effectively damps vibrations. Where it stuck through the decking, the hole was oversize by maybe 20mm all round. I had two struts connecting from the pole bolted to the brick wall that stopped the top of the pole from swaying around, these were just under the platform but were arranged so that neither the pole, nor the struts touched the platform. This way you could walk around without shaking the scope. It was pretty stable.

If I was doing this again I would drop the height to leave a space under the eaves, 40-50 cm, and build a small locker tucked away in there to hold the scope and mount when not in use, that way you won't have to lug it up and down and it will be reasonably out of the weather.

Make sure you have a good handrail and maybe some wire lines as well at knee level - I nearly fell off a couple of times in the dark when I was cold and stiff.

My cat loved it too, and often kept me company at night.