On that note, would a converted shipping container serve the same purpose, and in the op's situation, give added securityIn 1987 I applied to local planning to build a small office outside but close to a workshop we’d just bought.
Council refused permission.
Local business man advised building it anyway then if the come a knocking apply for retro. permission. I discounted that option but fabricated a rectangular (with an extra bit for a toilet) metal framework out of 100x 100 angle iron and put it on legs 200 mm long.
We then poured concrete into the angle iron framework for a base and the builder then built an office from concrete blocks on top, rendered it and put a tin roof on.
Not too long before the man from the council arrived to advise knocking it down or applying for retro. planning permission.
I explained it was a temporary building, not connected to the ground and could be lifted and moved.
He agreed and that office is still in use today.
KB.
Ps. You can just see under the toilet area the space from the ground level.