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I wouldn't (didn't) think twice about getting three-phase. OK, you need a three-phase main DB, but the cost is a drop in the ocean of a 400 m² build. Never mind heat pumps, etc., don't you want fast EV charging?
ETA: a three-phase supply avoids smart metering. 😁
That to me suggests the likely need for a three phase supply of 100 amps, or at the very least split phase at 100 amps.
Even with good insulation 400 square meters would probably require at least a 24Kw gas boiler, or say 12 Kw of electricity for a heat pump.
A guestimate of the larger loads could include
Space heating=12 Kw. No diversity allowable.
Cooker= 5Kw Very little diversity allowable, since it has already been applied to the cooker rating.
A couple of EV chargers=14 Kw. No diversity allowed in my view, both could run at full load for hours.
Water heating=3 Kw. No diversity allowable.
Consider the possibility, even a probability of a second shower, a larger cooker, and a load of portable heaters when the heat pump breaks.
Small appliances are unknown, but consider perhaps 4 ring finals with an assumption that one will be fully loaded and the other three much less so.
Even with good insulation 400 square meters would probably require at least a 24Kw gas boiler, or say 12 Kw of electricity for a heat pump.I reckon that's rather pessimistic. I've done some very rough heat loss calculations for a 400m² building (with a lot of assumptions and guesses of course) based on some L1A U values (0.3 for walls, 2 for windows etc), and come out with a figure of less than 6.4kW total for -5 degrees outside. If it's a self-build for someone interested in keeping their running costs down, insulating to something closer to PasivHaus standard could half that figure. That's just space heating so add a bit for DHW of course - but it's still a lot less than 12kW never mind 24kW.
Neighbours of mine have a house of just under 500 square meters and a 26 Kw oil boiler is only just sufficient and that is with an estimated heat input of another 5Kw from an oil burning cooker. The house is about 30 years old but was built with insulation far in excess of that then required.