Property dividing line

[Malagabob333]

I live in a semi detached house.My neigbours builder has started trenching for foundations for an extention and only just informed me of this new build.
I seem to think looking at the trench the finished face will extend to far on to my property.
Before his builder starts laying courses is there any way we can determin the property division line ie the finished face of his extention. can I go by the rainwater downpipe/gully ,or measure the full width of the two houses and half it?

[buildersmate]

The simple explanation is that no part of this extension can extend beyond the centre line between the two houses. This means unless your neighbour is putting a roof on it that has no protrusions, it will be impossible for the builder to build the roof without some part of it protruding into your side. Even if there is no guttering planned on the side that adjoins to you, have you ever seen a roof verge that doesn't stick out beyond the line of the brickwork below it? Even a flat roof has a bargeboard around it.
It is important that you sort this out now - it is very likely to impact your future ability to build such an extension yourself. If you is planning on draining the roof towards your side, definitely dissuade him - if you ever want a similar extension, both of you would end up with a 'contained' gutter at the bottom of a valley - these are notorious because if they leak, they leak into the actual property underneath.
In England and Wales all work around the boundary of a property is covered by the Party Wall Act and your neighbour should have served you a party wall notice. This does a number of things that you can read up about in this booklet.
http://www.communities.gov.uk/documents/planni ngandbuilding/pdf/133214.pdf
He hasn't done this so you could stop him dead until he complies. As well as the avoidance of protruding into your airspace, he has no right to protrude into your land below ground either. Generally this makes it impossible to build the wall on the actual boundary (because the foundations are wider than the wall they support and these must not protrude on your side either). Post again if you want further advice.

[Twenty20]

Not much to add to buildersmate's post except, didnt you see plans of this when planning permission was applied for?

[Malagabob333]

Twenty20.No I have not seen any plans, and have not been told in advance of this build.I have been away on holiday for one week and work started while I was away I came home to see trenches for footings.
The neigbour says she has planning permission,but I have been told that if she has applied I should have been notified by my local planning dept so that I could lodge complaints if any before planning application was granted.

[bednobs]

look on your local council website - if she has applied for permission it wil be on there

[buildersmate]

Its perfectly feasible that the extension does not need planning permission because the neighbour is using permitted development rights.

[Malagabob333]

I have found out that my neigbour has applied for and been granted building regulations approval and does not need to inform me of her intentions to build.
But have been told the max volume of the extention must not exceed 70 cubic metres.Does this meen the total area including the roof void/cavity/brick block work.
Looking at it at the moment Im sure it eceeds it.