With the lastest release of the Play Framework, the way multi-project builds should be created has changed. Unfortunately this change is yet to be documented. If you're following the docs but seeing unexpected (and unwelcome) error messages along the lines of object Project is not a member of package play then read on. I spent a while getting to the bottom of this, hopefully I can save you the time.

Short answer

Play's documentation on migrating from 2.3.x to 2.4 has the answer.

Long answer

In the past the correct way to put a multi-project build was to use the play.Project function in place of sbt's own Project constructor. the docs still say this is the correct approach but it seems as of version 2.3.4 Play integrates more closely with sbt.

Here's an example of how to convert a multi-project build definition that uses the more powerful *.scala format. I find for multi-project builds that the scala format is more useful but the same change should apply to the simpler build.sbt method of defining your project's build.

import sbt._
import Keys._
import play.PlayScala

object ApplicationBuild extends Build {

  name := """my-application"""

  scalaVersion := "2.11.2"

  version := "0.0.1-SNAPSHOT"

  lazy val common = Project("common", file("common"))

  lazy val admin = Project("admin", file(admin"))

  lazy val web = Project("web", file("web"))

You should now use the normal Project constructor from sbt and if you need it to behave like a Play application (use the Play directory layout and make the various bits of Play available) then you can include the PlayScala plugin using enablePlugin(PlayScala).