The house that wasn't there by Elana K Arnold

Alder has always lived in his cozy little house in Southern California. And for as long as he can remember, the old, reliable, comforting walnut tree has stood between his house and the one next door. That is, until a new family—with a particularly annoying girl his age—moves into the neighboring house and, without warning, cuts it down.
Oak doesn’t understand why her family had to move to Southern California. She has to attend a new school, find new friends, and live in a new house that isn’t even ready—her mother had to cut down a tree on their property line in order to make room for a second floor. And now a strange boy next door won’t stop staring at her, like she did something wrong moving here in the first place.
As Oak and Alder start school together, they can’t imagine ever becoming friends. But the two of them soon discover a series of connections between them—mysterious, possibly even magical puzzles they can’t put together. At least not without each other’s help.