How is buildSrc looked up in a multi-project build as documentation is a litte ambiguous here?

How is the buildSrc directory in a multi-project build looked up? Documention is ambiguous here. Is states: It is expected to live in the root project dir or rootProjectDir.

What I have is the following flat project layout:


and the following settings.gradle in checkout-root: = 'projectBuild'
rootProject.projectDir = new File(settingsDir, 'projectBuild')
  includeFlat 'projectA', 'projectB'

projectBuild contains the build.gradle for the multi-project build.

After trial and error I found out that Gradle expects buildSrc to be in Settings.rootDir and not Settings.rootProject.projectDir .

Is this correct and if it is, can this be changed?