"When George showed her the plans for the studio it reminded her of Jamie’s plans for that machine to catch Santa Claus. But there it was, at the far end of the lawn, foundations laid, five rows of bricks, window frames stacked under blue plastic sheeting."
More details from Mark Haddon's engaging site here.
The only other recent book I can think of which features something similar is Beyond Black by Hilary Mantel in which a shed does indeed play an important role. If you can think of any other fiction in which garden offices/studios/sheds play a part, however small, please add your comments below.