Thinking of building a garden office

Can’t afford a 10K build and was thinking of building a single story SIPS structure about 5m x 2.5M using Kingspan TEK panels or similar thinking I can cut them to size myself and cut out spaces for windows etc.

Few questions:

The site is next to a retaining wall (about 4′ high) in a tiered garden (sloping down a hill). Would I need some special SIPS panels to put next to this wall (foundation grade)? Or could I just put cladding on normal SIPS panels before putting them in place – I…

Thinking of building a garden office