Based on reading the planning application, it seems that a terminal under 500sq/m in size doesn't require planning permission and consultation. The new terminal is 486 sq/m.
They're building it as small as possible to force it through the planning system as easily as possible.
If flights from Carlisle are a success, then they'll expand as needed, and should stand a better chance of getting planning permission, as it would technically be an extension rather than a new build facility.
So far everything is being done as cheaply, and with as much government money as possible. Very little risk to Stobart if it fails.