Looking at a map of the Teesside area, there are plenty of places where houses could be built that are closer to the centres of Stockton, M'boro & Darlo, so I'm not sure it actually is prime building land. Even if it was, the airfield is so large that Peel could flog off large parts of it and still have enough room to run a busy airport (and build a 747-size hangar if needed).
Is the airfield south of the runway still owned by the airport?