The reading age was around 10 years. They were probably written in the 80s, maybe the early nineties at the latest. They were about an old house that some family members (grandparents or great aunt and uncle) lived in, and either two or four children stayed there and mysterious things happened.
The only part I can really remember is an owl coming down the chimney.
They weren't strongly focussed on magic, but there was a supernatural element to them.
I have a feeling there were four books in the series and there was one for each season, such as "Winter in..." and then the name of the house, such as something hall.
Argh! I'm having trouble explaining.