Hi,
If what your neighbour is proposing falls within his permitted development rights then unfortunately the council have to approve it providing he sticks to the limits. The general limits are:

Semi-detached & terraced property -
3m deep, maximum eaves height of 3m and a total maximum height of 4m

Detached property
4m deep, maximum eaves height of 3m and a total maximum height of 4m.

There are many other rules as shown on the planning portal
http://www.planningportal.gov.uk/permission/commonprojects/extensions/miniguide

I would suggest discussing this with your neighbour, we negotiate with planning officers on this scenario on a daily basis. It is normal practice for an extension to build right up to the boundary position between you and your neighbour.
However if an extension is to be Permitted Development there can be no encroachment over this boundary line in plan or elevation, if there is then a householder planning application will be required where the planning officer will take into consideration a neighbours amenity and enjoyment of their property.Planning Officers can use the 45 degree rule where a line is taken from the centre or closest edge of a neighbours nearest habitable window and if the extension crosses this imaginary line in plan or elevation then they can reject the application. Planning Officers normally only apply this rule if there is a strong objection from a neighbour.
If a householder planning application is lodged by your neighbour you will be notified as part of the application consultation process and at that stage you can support or object an application.
Generally it is considered that a 2200mm height at the eaves on a proposed extension on a boundary is considered acceptable.
With Permitted development this height can be up to 3 metres to the eaves and 4 metres to the highest point of a pitched roof, flat roofs need to be a maximum height of 3 metres on a boundary.
Before you object it is a wise approach to discuss with your neighbour and let them know that an extension of this height/projection on a boundary will spoil your enjoyment of your property and if they are a reasonable neighbour will adjust their plans, if you propose to build a similar extension to your neighbour then this problem is resolved.
Hope this helps
Pete Watkin
Partner
Aztech Architecture Ltd

Look out for a public highway. Its got to be more than 5m away (inc footpaths) or it will need planning. Many people have fallen foul of this even when within permitted development

Best thing is to contact local building authority and ask there advice as the will give you the correct answer.

Hi, first i have good chat to neighbour, you dont wont to have bad feeling if poss it makes life bad, if you feel very strong about it ask the council,but last thing you need is a war good luck colin.