What's behind the door?

Working from home today, I was surprised to see a cherry picker appear over the roof of the disused chapel at the back of our house. Glad to see they were fixing all the roof tiles at last. It's a lovely old building - I'd love to have the money to buy it. We've joked with the children about having it as our house, taking down our fence and having our current house as the "shed". Or perhaps just filling the chapel's insides with many slides and water chutes.

In the seven years I've lived here, I've never once seen that door open, despite seeing it every day. I had to overcome a very strong urge to go and ask the man if I could nose inside - to plan where those slides would go...