This makes interesting reading Graham.  I do remember Andrew Seedhouse confirming that there were no [i]significant[i] growth plans along the trans wilts corridor other than in Swindon.  Some of the figures for rest of the North and West Wilts areas look pretty scary though.  My concerns would be:

1.  How the hell are the roads going to cope if we havent got decent public transport to use?
2.  Where are these houses going to go?

I know in Chippenham for example that most of the brownfield sites have been or are being developed, they would have to start buildong again on greenfield land.
I have tried searching about on the Wiltshire County Council and North Wiltshire District Council sites but cant see anything in the plans about where the development might take place other than on land I already know of.

Hell, at this rate Chippenham, Melksham, Corsham and Trowbridge will become one place in about 20 years! 

Having just written "it's possible they could all go in one designated area", I worked out what that could look like:

Bratton and Edington New Town

To meet the extra housing needs of an expanding population in the next 20 years, an additional 11,000 to 12,500 new homes are to be built around the villages of Bratton and Edington in West Wiltshire. The population of the new town will rise to approximately 30,000 by 2026, so making it comparable in size with Trowbridge today.

Access to Bent (as the new town will be called) will be off the Westbury bypass, currently in the planning stage.  A flying junction just below White Horse Hill will lead to a 3 mile access road running along the valley just to the North of Salisbury plain.  Terraced housing on the edge of the the plain will allow maximum housing density to be achieved while at the same time giving most new residents spectacular views across the valley of the Avon and up towards Pewsey Vale.  Rail access will be at nearby Westbury station, although if the population growth exceeds expectations, it's possible that the Department for Transport could approve the reopening of Edington Station with a limited London commuter service from 2021.  This would need additional financial support from the County Council.

Schooling for primary children will be in the town, with secondary children travelling by bus to their parent's choice of Melksham, Trowbridge or Warminster. Medical facilities are already available nearby in major hospitals at Bath, Salisbury and Swindon, and a new doctor's surgery will be included in the plans for Bent. Good shopping facilities are available at Trowbridge, to be supplemented by a convenience store, a hairdresser and a selection of take aways in Bent itself. Jobs will be at Swindon, Chippenham, Melksham, Trowbridge and Warminster; it's anticipated that there will be little new employment offered within Bent itself, as it's not historically an area of high employment apart from in agriculture, and planning applications for major factory units may not be acceptable to the local population.

In 1821, one in every 100 peple in Wiltshire lived in the Bratton and Edington area, but that dropped by 2001 to just one in 350.  These proposals for Bent will help restore that historic balance and more and put Bratton and Edington back on the map.

Author's note - this is a spectulative proposal without foundation in fact - or is it?  The Reginal Spatial Strategy is calling for the number of additional homes suggested to be built in West Wiltshire excluding Trowbridge, and options such as the one outlined must be the sort of thing to be considered - if not at Bratton and Edington, then elsewhere in the area. Road and rail transport, schools, hospital, employment and building plans suggested are based on current practise for towns of this size.