Heating / ventilation for a new room

[newbiechauf]

Hello,

Here I am doing an extra bedroom with ensuite bathroom in my cellar semi-buried (about 15m ²).
This cellar is however, like all cellars: wet and cold in mid-season.
So I want to predict from the outset a very good ventilation system. This is where it gatters because I seem to exist several solutions and I do not want to embark on a new energy system ...

Currently my heating is an old gas boiler (18 years) which also makes ECS and which costs me 1500-1600 € of gas per year (7 radiators and 2 dry-towels this without counting the new ones that I intend to create for my room with the -ground).

I imagined different solutions but I do not know which would be the most suitable:
1. I connect the new room on the current heating network and I add a very good VMC for the room and the attached bathroom. I'm afraid that this solution will quickly come back very expensive because the heating will probably be lit sooner than now ...
2. I make independently of the current system an isolated heating / dehumidification system:
at. with a PAC air / air and a split allowing dehumidification. (In this case I guess you should not add VMC?)
b. with an air / water heat pump and a fan coil for dehumidification.
3. I make a complementary system that adds to the current, with a completely stupid electric heater in addition to heating by the general network. I'm afraid it will be very expensive in electricity.
4. I change all my heating system + DHW by a PAC air / water but what will come back to the same problem of the ignition of the heating in advance of phase because of the cold of the basement in mid-season ...

By browsing many forum, I understand that the energy supplement by PV Panels were not very profitable ... however many of the posts I read are from almost 10 years ago. I guess things have not changed much since ...
What would you advise me?
Thank you beforehand,
Cdlt,