So not really a storage room, more like a utility room where you can do the clothes washing and drying etc?
This really is habitable space and would fall under reg's.
There is nowt stopping you building a conservatory type building as long as you maintain an external spec' door separating the main dwelling from this new building.
But why bother as it will be freezing cold and cost a daft amount to keep warm. Bite the bullet and build an extension where you can have the drains and electrics etc, signed off with peace of mind.
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