When I was in about 3rd to 6th grade in the early 1970s, my favorite book in my NJ public school library was a hardcover chapter book about kids on summer vacation, possibly in upstate New York, who get involved in some kind of mystery involving a neighboring house they call the Chateau. I think a diamond necklace came into it, and I’m pretty sure the children of one family joined forces with a boy from another family (maybe living in the Chateau?) to solve the mystery. Either Chateau or Diamond Necklace might have been in the title, but I’ve had no luck Googling for it. It was an old-fashioned story at the time, probably could have been written any time from the late 1930s to the mid 60s. There were illustrations, but a limited number, and I think they might have been listed by caption after the table of contents. I think it was the first time I had heard of a “Porte cochere”, which I had to look up. I think the binding was red, and may have had an imprint illustration of a country house on the cover — but I could be making that up!
I loved it and probably read it four or five times, but these scraps are all I can remember! I’d dearly love to find a copy.
Perhaps GONE-AWAY LAKE by Elizabeth Enright?
Elizabeth Honness: The Mystery of the Diamond Necklace (1954)
Could it be Mystery of the Diamond Necklace by Elizabeth Honness?
Description from an Ebay posting: “Mystery in an old chateau across the road is reopened by the Duvals. They make friends with Mavis Duval, but the plot thickens when a jewel thief starts prowling and makes a play for the diamond necklace, a long lost heirloom hidden in the Duval chateau. The kids help beat him to the draw, and for a still happier ending there’s romance between the Clovell’s newsman uncle and Mavis’ Tante”
Also found an image of the vintage cover in red: https://br.pinterest.com/pin/254734922661784426/?d=t&mt=login
The Chateau Holiday by Winifred Mantle? There are at least five books in this series about two families who join together and have adventures…so maybe if it wasn’t this title, it was one of the others.