ᐅ Heating solution for attic conversion

Hello everyone,

I am looking for a suitable heating solution for the conversion of my attic. Here are the key details:

- End-terrace house, built in 1998, 10cm (4 inches) external wall insulation
- The attic is currently unfinished and will have dormer windows on both sides of the roof, then divided into two rooms, each approximately 20sqm (215 sqft)
- The two rooms in the attic will be children’s/teenagers’ bedrooms, meaning they will be used intensively—and therefore heated—for many years
- Roof insulation: 16cm (6 inches) mineral wool insulation between rafters, 14cm (5.5 inches) wood fiber insulation on roof deck
- Heating: gas central heating (built in 1998) with buffer tank in the basement; ground floor and first floor are heated via radiators (no underfloor heating); additionally, a fireplace on the ground floor; despite its age, the heating system is in excellent condition and very energy efficient
- My preferred solution so far for heating the attic: water-based underfloor heating connected to the central heating system/buffer tank in the basement; problem: there is currently no heating circuit or conduit running to the attic; this would require drilling through three ceilings (basement ceiling, ground floor ceiling, first floor ceiling), which I would prefer to avoid.

Does anyone have ideas or tips for
... a simple way to still implement the water-based underfloor heating?
OR
... alternative heating concepts?

Best regards,
Matthias

My Plan A would be to add an extra floor with radiators, avoiding an additional circuit from the basement going up.
Plan B would be to add a split air conditioning system as well, to be able to cool in the height of summer and heat in winter (with photovoltaic support?).