Sorry Durham, I just can't see this being a logical, or practical idea. The UK is rapidly running out of space for building houses on, just look around you. Houses are being crammed onto smaller and smaller bits of land. In my area, they've ripped hundreds of trees up and built on the land. As a consequence, that land now floods and the houses can't be insured. Add another 500,000 houses onto what's already been approved. Where? What land is left to build on, let alone stick 1 million trees on!
But I'll leave you to continue to champion the whimsical fancies of Corbyn. You'll justify his mad ideas, no matter what.