This was a full application, not PD. The 50% is their own rule for what they do/do not allow when it comes to extensions.
Ours has a similar greenbelt policy. Normally it's framed as "disproportionate development will not be allowed in the green belt, we take this to mean 50%"
In reality, they're flexible depending on the circumstances of each individual house. My neighbour has somehow increased a 125% increase (i.e his house is now 225% of its original size) and yet I've had to argue the toss with them to get a 69% increase. I went in with a 90% proposal, had an outright rejection and settled on 69%.
What I'm getting at is that you should talk to someone that knows the local policy, and can look tat the individual circumstances of your house.
In the end I've devised a scheme of maxing out PP and PD, and sequencing the builds in the right order in an attempt to get the best of both sets of regulations. Maybe you can do something similar?