In medieval times in England the standard unit required to - barely - feed four households on a farm was called a Hide, and it's roughly 120 acres.

Assuming we're much smarter now - hmmmm - we can perhaps get the grain requirements of a family down into an area of maybe five acres instead of thirty. (That's possibly optimistic. Modern farming returns grain yields of 20:1 but is very intensive, and also allows fields to lie fallow every few years, which is only possible if agriculture is collective.)

Five acres is roughly two and a half football pitches, which is a little bigger than most people's yards.

The permaculture people claim you can live off a much smaller space than this. And if you build some greenhouses to concentrate energy and keep bad weather out you can - perhaps. But you won't be able to do it by growing cereals, which are possibly the most energy intensive of all vegetable crops.

And other crops really aren't all that calorific. If you want to eat a minimum of 1500 calories a day, you need very, very big plates of vegetables, or - ideally - some other source of carbs.

So realistically the most you can expect from urban agriculture is a bit of fill-in, and perhaps some barter. It will certainly help, but mainstream agriculture really does need to be collective and large-scale for maximum social and economic value.