Stripeymama,
the cover (without dustjacket, don't remember it having a dustjacket) was green.
The descriptions of the house, false rooms, priest holes was very exciting. The treasure was lots of gold plates, chargers, candle sticks and think they were on steps going down to the dungeon. I'm not even sure there were baddies - maybe just lots of treasure. The grandfather adopted her.
Totally clutching at straws - the typeface was not 'childish', no pictures. Fairly sure it was written by a bloke and remember no other books he's written.
I'm sure I only read it once which means it would have been at an elderly relatives house. It wasn't one I actually owned.