I can't help thinking that knocking down buildings to clear land would be a pretty huge undertaking for relatively small returns. I think rooftop gardens, window boxes and planters would be the way to go. I'd see any flat rooftop used for food production (if you could plant grass up there then rabbit farming would be feasible, and their poo could be used to manure your vegetable gardens). I guess chicken farming would also be feasible too.
I guess that you would need to look at what you are growing - those plants that require deep soil for their roots would need to be farmed in parks and vacant lots at ground level, while those plants that could manage in shallower soil would be up on the rooftops.
On an individual level I think everyone living from the first floor up (second floor up to you Americans) would have window boxes planted with vegetables and herbs (we have a couple of herb plants in flower pots on our kitchen window sill today) - I think that planting on the roof and first floor windows would provide additional security for the individual' harvest - and stop casual filching of carrots etc.
I found this rather neat site when I looked at this - the first link (to a Chicago "victory garden") is pretty interesting - Chicago also publishes a PDF giving some basic engineering details of what is required.
http://www.cityfarmer.org/subrooftops.html
Some of the links there are fascinating - I was taken by the statistic that a survey of Singapore covering about 20% of the whole found 212 hectares of potentially usable land.
One of the pieces also talked about growing mushrooms in basements - evidently this can be an ideal environment for mushrooms so that's another potential area that could be utilised.
But the neatest item was, I thought, this one from Egypt - if you look they are using what look like plastic drainpipes (with large holes drilled in them for the plants). I couldn't help think that if you built a frame you could actually mount these on the wall of your building (so no need for a roof at all). So all you'd need for harvesting is a ladder.
http://www.islamonline.net/English/S...rticle03.shtml
One of the the links had a picture of an abandoned "El" in NYC - The High Line - with weeds growing up between the tracks. Now if this doesn't have potential for growing crops then what does?
http://www.nyc-architecture.com/CHE/...heHighLine.htm
Malcolm