Against planning

Planning is the right of local authorities to prevent others from building what they like on their own land. I am against it because:

it involves force
it restricts the supply of housing leading to higher prices
(by the creation of a Green Belt in London) it encourages development beyond it, so presumably despoiling another part of the countryside
(by the fact it is managed in an inconsistent way) it tends to lead to stop-start phases of building
most of the really nice buildings and parts of London were built without the benefit of planning at all
it deters people from experimenting with new designs - like they do in France and just about everywhere else
it takes the little guy out of the building business
it leads to longer commuting times

Questions

How does planning restrict the supply of housing?

Why don't they?

I'm not quite sure. I guess it is partly because they don't want to have to go to all the extra expense of building new schools and other facilities and partly because residents ie voters don't want new building to be built anywhere near them. NIMBYs as they are known, NIMBY standing for Not In My Back Yard.

But if we scrapped planning that wouldn't that mean the concreting over of the South East?

I dislike the term "concreting over". It's not true. The most accurate term I can think of is "urbanisation"
I doubt whether the end of planning would mean complete urbanisation. There are plenty of green spaces in those parts of London that were built without the benefit of planning legislation (think Hyde Park, Wimbledon Common etc). How, I am not quite sure.
People like to live near bits of green. Therefore, there is an incentive for developers to provide it, though this would probably require a super-landlord
Even if it did (mean complete urbanisation), so what? If people convert grass into concrete that can only be because they value concrete more than grass. Or, in other words, because it makes their world a better place

But without planning what's to stop your next-door neighbour building some eyesore or even a petro-chemical plant?

In the case of the petro-chemical plant, cost. Residential land tends to be expensive, while industrial concerns prefer to build on cheap land. Having said that there are some people who might prefer to live near petro-chemical plants - the workforce, for instance
In the case of the eyesore, nothing, at least, not initially. What this probably requires is a super-landlord
It's worth noting that in those areas of London built without the benefit of planning there are few eyesores. Why, I am not quite sure. Possibly super-landlords such as the Grosvenors. Possibly social pressures

How does planning prevent people from experimenting with new designs?

Because, it so difficult to obtain planning permission the main concern of the companies that build houses is securing permissions rather than the quality/inventiveness of the houses they actually end up building. Probably.
It's not just planning. Building regulations have become so complex that once architects have come up with a design that works they tend to stick to it

How does it exclude the little guy?

In case you are in any doubt about this just ask yourself how many people design and build their own houses. Not many.
The planning process is long-winded and expensive what with all those hearings and plans. The little guy cannot easily afford to submit plans that might be rejected so he is usually better off not bothering. He is also unlikely to understand the "tricks of the trade" that the big boys (such as Tesco) understand