Garden Room / Home office

I have a DIY garden room with 80mm (wall) and 100mm (roof) external woodfibre insulation, with DIY triple glazed window and French doors and reasonable amounts of permanently open ventilation (gaps around the doors!).

It is used daily as my office and at weekends for hanging out.

In the last 30 days it has averaged 5kWh/day electricity consumption on the 2kW resistive heater.

In practice this means around an hour at full blast and then trickling for the rest of the day.

If I wanted more economy I would add internal insulation, close up the gaps and use a single room heat pump/HRV instead of a resistive heater.

Glazing will make a big difference. If you have a window that gets direct sun (I don't) then the room will stay warm during the day without heating, just needing a boost in the morning. Obvs that may cause overheating in summer so consider shading. The risk of overheating from a poorly insulated roof is greater.

So in your position I would max out the insulation spec as that's awkward to increase later, fit closeable ventilation, and think about solar gain and shading.

Remember that a small building has a massive surface area. Mine has maybe 25% the surface area of the house even though it's only 7% the floor area.