This is going to make me unpopular with all those who see their house as their home and their castle and their pension pot...
The problem is that there is a very serious inequality and unfairness in the distribution of the national housing resource. Those with the money have collared more than their fair share. And having bought the largest pad that their money (and that borrowed from the bank) can buy they go and extend it still further. More space and bigger pension and that is without going into the status and appearances and keeping up with the Joneses. Messrs Pickles and Boles who conspired to increase the allowance to be added to houses without permission will not like this.
I firmly believe that houses should be left alone. Every house should be brought up to a high level of energy efficiency either before it can be sold or within the next ten years. At that point interfering with the fabric and the high levels of air-tightness will reduce the energy efficiency. The materials used in the extension will have an energy cost (25% in the materials and 5% in the construction work). The new space will need extra energy to be heated (the remaining 70%) and putting minimalists aside, the space will attract stuff. The outside space for growing, drainage and bio-diversity will all be reduced. The end product produces a higher pension pot by becoming less affordable to those just wanting a house to live in. If sustainable development has three pillars of environmental impacts, social inclusion and economic fairness (and efficient use of resources) then house extensions will not meet the presumption in favour of sustainable development in the Framework (term now being used for the NPPF).
So LPAs should be removing permitted development rights when granting permission for new housing and even make Article 4 Directions to have the same effect for existing residential areas. Local plans and neighbourhood plans can include policies to indicate when and where house extensions would be permitted and subject to what conditions. Whilst the increase in the allowance for permitted development was intended to simplify the planning system and relieve over worked planners, actually eliminating or severely reducing the number of house extensions would be more likely to have those benefits.
The hardship suffered by those who really need extra space would be alleviated by a greater movement between properties in order to meet functional requirements. I cannot think of a system that would prevent people buying properties larger than their household needs but would repeat the plug for co-housing where all those facilities (and more) that do not fit in small and unextended homes are provided on a shared basis.