Independent power supply.
Temperature control, humidity control.
Insulation aim for 120 mm thick overall wall thickness, the thickness of high end outbuildings.
I used 75mm cavity, cost being the reason.
See DIY tank stand pics in DIY.
You or somebody must be able to calculate the space needed on site, guessing online I would say be careful. Find an engineer or carpenter.
Follow link to Flickr page, all construction pics have a brief but concise paragraph.
Connection to the meter must be carried out by a qualified electrician.
Armoured cable, thickness depends on distance to meter, other options available.
Roof, thickness as per walls, insulated.
Floor, concrete mix.
Door, airtight, you will waste heat.
Without heating system it is impossible to maintain stable tank temperature. Tested.
It's all still fresh, my mind has done little else for twelve months of planning.
You must be capable of overseeing the project or outsource.