The zone would exclude A and B roads, red routes, and private roads. The map below shows Earlsfield (yellow areas are already 20 zones).
The council says:
"This consultation is whether to introduce a 20mph limit across the borough on those residential roads that are currently 30mph, but excludes main traffic routes.
No additional speed reducing features, such as road humps, are proposed.
In some roads, where the existing mean traffic speed is above 24mph, and to comply with Department for Transport regulations, white road markings in the shape of a speed cushion (ghost cushion) may be required. The idea is to give the impression to drivers that traffic calming is a feature and they should moderate their speed accordingly. Whilst this may not have the same speed-reducing effect as a physical measure it has been used in other areas with some success."
You can respond to the consultation here.