Mushroom house vs shed

An interesting question raised by a reader of the Sunday Telegraph who has a 'mushroom house' in his back garden - it's stone, under a grass bank and rather damp. Should it stay or should it go? David Snell, contributing editor at the interesting Homebuilding & Renovating magazine and author of Building Your Own Home, gives a couple of suggestions but then asks: "Is it worth it? You could probably buy or build overground sheds or outhouses for a fraction of the cost." While normally I'd endorse any shedroute, it does seem a shame to lose a mushroom house.