Cooling is the easisest to add, although this may apply more to those outside the UK. Look up "Solar Chimney". You can make one of these from some wood, glass and black paint. Once that's up you have a chimney,driven by the sun. When it heats up when the sun is shining, air moves up the chimney. You have that draw from the room you want to cool. Next a 6" hole is bored from that rooms floor, at 45 degree angle and long enough to clear the room for a few feet. Where it does another hole is bored straight down about 8 to 12 feet or so. With this open the room with the solar chimney will pull air through here. Four feet down it's always 50F no matter where you are. So, all you have to do is vent the hot air out and pull in cool air from underground.