This is another one from the test.
The correct phrase is:
К дóму мы реши́ли пристрóить сарáй.
And the wrong answer was 'подстрóить'.
My dictionary isn't very clear but it seems that пристрóить has the sense of 'build, add on' whereas подстрóить has the sense of 'build on, add a wing to a house'. Since your barn would be separate from your house подстрóить is the wrong answer. This seems quite a subtle point. Can anyone explain to be a bit about how I could have told this from the При and Под prefixes?