The book I cannot remember is from late grade school or Junior high (7th grade). It was a spaceship, I think in the backyard with a strange builder or helper to the kids building or traveling on it.
Stump the Bookseller
New format, same book stumper solving.
Possibly The Wonderful Flight to the Mushroom Planet, by Eleanor Cameron. David and Chuck build a spaceship for a trip to a secret planetoids, aided by the mysterious Mr. Bass.
Could be The Space Ship under the Apple Tree, by Louis Slobodkin.
The Wonderful Flight To The Mushroom Planet https://www.goodreads.com/book/show/527747.The_Wonderful_Flight_to_the_Mushroom_Planet or The Spaceship Under The Apple Tree? http://www.slobodkin.net/books/target98.html
The Wonderful Flight to the Mushroom Planet?
Possibly Eleanor Cameron’s The Wonderful Flight to the Mushroom Planet.
Rusty’s Spaceship, by Evelyn Sibley Lampman?
Rusty’s Space Ship, by Evelyn Sibley Lampman?
https://www.goodreads.com/book/show/11390330-rusty-s-space-ship