It would have to be a plywood bungalow (also known as shed), and the land would have to be free (and not green belt remember) as they only have £5400 per house at £2.7bn. A more accurate costing would be £50-100bn. All this while giving everyone £72 a week.
If one ignores the fact that houses will actually be sold and not given away for free. 2.7bn is plenty of initial capital to start building a lot of houses.
Under the Green manifesto people may get 72 a week but they won't be getting welfare, child tax credits, unemployment benefits, housing benfits, job seekers allowance, or pension to the same degree as is now. Give people a living wage right off the door and then you can do away with all of the complexity and overhead of state handouts Ina piecemeal fashion, improving efficiency and reducing taxation requirments.
It is little more than a form of negative income tax which has lots of advocates, even from the far right.