ESP8266 based, why not. But I have been looking at the sensors you can get, and bought a few.
Are all CO₂ sensors crap?Well, I tried a few, including this CJMCU module (CCS811 based) and this AppliedSensor module.
Sorry to say, but both are crap! And the detail is in the small print - they estimate CO₂ from other sensors, and are very easily confused. I have some isopropyl alcohol and you use that anywhere near them and they quickly say 18000ppm CO₂. Even without such factors they swing wildly all over the place.
It seems you need an NDIR based sensor, and I found this one which seems to actually work! This SCD30 sensor seems to be the business, with CO₂, temperature and humidity (floating point!) via I²C.
Next step - sort a suitable display and box.
I can then use the temperature to control the air-conditioning, and the CO₂ to control extractor fan speed.