A small, well-insulated, heavily glazed building heats up fast and has almost no mass to absorb it. Three measures fix it and all three are decided before production.
Shade the glass from outside
Once the sun is through the pane the heat is inside. An external overhang, shutter or screen intercepts it; an internal blind converts it to radiant heat and warms the room anyway. The difference in effect is roughly threefold.
A fixed overhang sized to the summer sun angle blocks the high July sun and lets the low December sun through. That is a drawing dimension, not a product.
Give the roof some mass
The roof takes the most radiation and a lightweight build-up passes it straight through by mid-afternoon. A denser insulation — wood fibre rather than mineral wool — delays that transfer by hours, which is enough to move the peak past bedtime.
This is why the loft is always the hottest place in a compact house and why it is the element worth upgrading if the budget only allows one.
Purge at night, close during the day
The Mediterranean habit works everywhere: open up when the outside air is cooler than the inside, close and shade when it is not. In a small volume the air changes quickly, so a two-hour evening purge genuinely resets the house.
This needs an openable window at each end and a rooflight over the loft. A house with only fixed glazing cannot do it at all.
Cooling, if you need it
An air-to-air heat pump reverses in summer, and in a compact house the smallest unit is more than enough. It costs electricity at exactly the time solar is producing most, which makes it the natural pairing with a roof array.
Size it against the shaded, purged house rather than the unshaded one. Cooling a badly shaded building is the expensive way to solve a drawing problem.
Where the appliances are
A fridge in a sunlit corner, a router in a closed cupboard and an inverter in an unventilated void all add heat and all shorten their own life. In a small volume the sum is noticeable.
Give each a ventilated position. It costs nothing at the plan stage and it is one of the reasons some compact houses feel warm with no obvious cause.