Building an underground store for produce in UK

[duncang]

Good evening self-suffientish-ers!

It's been a while since I have been on here, apologies for that, but in this hour of need there was only one place I could go for help!!

I run a small artisan bakery and we always struggle for space... I have some land we have permission to use, and we already have a geo-dome on it where we grow our chillies, and soon to have our outside clay oven there too...

I am looking to remedy this issue of storage by building an underground store/dug-out/pantry.

Here are the facts:

Ground: Soil + clay

What I am looking to store:
Veg such as potatoes, onions
Fruit, mainly apples

Size
Approx 5sq metres inside

Questions:
1. Has anyone built something of this kind in the UK? If so, share your thoughts, photos, ideas please!
2. Planning permission: needed or not? I see it as a non-permanent structure, so am hoping not!
3. Ideas on materials. I have seen a few posts (mainly from the US) about building with sand bags, but interested in any other ideas from people, especially those who have done something similar. Focus is on something cheap and something eco!

Any ideas at all, be amazing to hear your thoughts!

Best regards,
Duncan