If you want to understand the bigger picture, start by looking here.
http://www.planninghelp.org.uk/learn-about-the -planning-system/regional-spatial-strategies/
This website is run by the Campaign for the Protection of Rural England, so it is actually promoting the reverse of what you are wanting - but there isn't a better Government site that explains what has been happening in the last 4 years.
The general policies haven't changed - towns and cities have settlement boundaries and within these, appropriate development is permitted. Outside them is countryside where it is against policy to inject new developments. In addition Councils have always had quotas, but there has been less obligation to ensure completion of quotas. Councils had to show development sites in their 10-yr Local Plans - mostly within existing settlement boundaries - but not always. This would show where most new development would occur.
New one-off houses as infill were always part of the plan, and although Planners would not know eactly where these might crop up, an allowance for them was included.
As others above have pointed out, some Councils (particularly rural ones in the south) were dragging their heels and few houses were going up - even on named sites. Couple this with Government's newly found desire to increase the housing density on sites (no. per hectare).
In short, you've fallen foul of changing policies. new houses are required, but not at a price of destroying lots of farmland in rural areas. Your plot is presumably in countryside, and whilst you might have got permission as a one-off, even against the policy of no new houses in countryside, if there is now an additional policy to limit to specific number in that area and the number is reached, then you are asking for something that is in contrave