Installing water pipe to outbuilding - how to handle insulation/cold so it won't burst/will be usable in winter?
Typically, you need to bury the supply pipe below the frost line until it gets to a heated space.  The local building authority should be able to tell you how deep for your climate, but it's probably in the neighborhood of 3 feet/1 meter.

However, you're not bringing it in to a heated space.  So you have a few options.


Use a freeze-proof hydrant.

Enclose, insulate &amp; heat a small cabinet/closet for the water access.  Heat could be as simple as an incandescent light bulb &amp; thermostat outlet depending on the size, temperature and insulation.  If you go this route, you should have a way to monitor the system - having a lightbulb involved (even if it isn't the primary heat source) that's visible from outside can give you an easy visual indication that the system is on.  And make the system easy to drain if you expect it to be out of use for an extended period or if you're prone to power outages.


You mentioned placing a water heater in the outbuilding - if it's a tank type heater, that could work well for the heat source in option #2.

More on additions to the question...
If the void and area above it aren't heated, then I think you're still in the same situation and the pipe will need to be installed to the required depth below the bottom of the void.

The problem with your latest above-ground update is that the freeze-proof hydrant drain is never open to the source side of the plumbing.  It's only open to the downstream side when the valve is closed.  So you'd have to have some other drain mechanism below the frost line.  

Since you have that 150mm void below the floor, is it feasible to simply put a drain valve down there and not mess around with the hydrant at all?  When you're done, turn the water off at the house, then reach down and open the drain for the whole system.  I know it's not much space, and you have to think about where the water goes.