Question 1: I plan on building a small garden shed, 5m x 9m, and I'm probably going to use cement pillars. I'm wondering how deep down they need to be buried? The pillars at the back of the shed will be about 1.8m above ground and a minimum of 2m above ground at the front of the shed. A layer of cement then as a floor I.e. around the pillars thus adding stability.

 

Question 2: In addition to this, I want to build a good, sturdy trellis also using cement pillars. They should be between 1.7 and 2m above ground. Again, how deep should they be buried? 

Hi, most builders I have seen put in a concrete foundations about 60cm square for each post. Then they stand post up on the foundation and concrete around it. This way if you get the foundations all at the same level, then easy to get the posts the correct height. Though in your case the back foundations would need to be 20cm lower than the front foundations. Depth for foundation around .4m based on if you have solid soil. If soil still loose, then may need to go a bit deeper