Saffron you could have a flap built into a window leading to a conservatory so you can allow them freedom this way but in a safe and controlled environment,many get standard ready made runs and adapt them to suit their cats needs ie -heat pad/lamps-tv/radio/toilets in certain place,etc so have a looksy around or visit local breeders catterys to get ideas and take it from there,give your oh a new hobby-proper man/woman stuff-diy