Roger wrote:
Having written the above I went for a brief google and found on an
architects site:

"In this document any reference to the volume of the house means the
original as-built volume including any extension built before 1st July
1948 measured externally (in cubic metres). Extensions built after this
date do not count as part of the volume of the original house.
Extensions built as Permitted Development after 1st July 1948 count
cumulatively towards the maximum size permitted."

I read that as including any demolished part but the draftsman could of
course not have thought that far ahead and in any case it is not an
official site.

I followed your example and my googling turned up the London Borough of
Waltham Forest, who have a nifty step by step householder's guide to
permitted development at:
http://www.lbwf.gov.uk/index/environ...age2bguide.htm

Three important definitions given there a
# The term dwellinghouse is used to mean the house as it is before the
development is carried out. It does not include any separate buildings
such as a detached garage, even if built with the house.
# The term original dwellinghouse is used to mean the house as it was
first built or its size on 1st July 1948, if it was built before that
date. It does not include any separate buildings such as a detached
garage, even if built with the house.
# The term resulting building is used and means the original
dwellinghouse PLUS any extensions added since 1st July 1948 PLUS any
buildings bigger than 10 cubic metres, which are closer than 5 metres
from the dwellinghouse and are within your garden PLUS the proposed
extension/building.

So it seems that the outbuildings wouldn't have counted as part of the
original volume anyway, even if built as part of the original
development, but if built after 1948 would need to be counted as part
of the final volume.

After punching in all the details, the site gave the following advice:

Your proposals will be permitted development provided:

* the resulting building does not exceed the original dwellinghouse
PLUS 50 cubic metres or 10% of its volume, whichever is the greater
(subject to a maximum addition of 115 cubic metres);
* any part of the building which is enlarged, improved or altered
is not closer to a highway than the original dwellinghouse, unless it
will be at least 20 metres from the highway;
* the part of the building which is enlarged, improved or altered
is not higher than the height of the highest part of the roof of the
original dwellinghouse;
* the part of the building which is enlarged, improved or altered
is not more than 4 metres in height and is within 2 metres of the
boundary of the garden of the dwellinghouse (this does not apply to the
insertion, enlargement, improvement or other alteration of a window in
an existing wall); and
* not more than half the area of your garden (excluding the area of
the original dwellinghouse) is covered by buildings.

If I'm reading this correctly, then I reckon I'll be OK for permitted
development. Bloody useful tool from LBWF!
--
Rob