Guidance on evaluation and criteria used to assess the
conservation/historic/design value of parks and gardens gardens

Dear tomlm-ga

English Heritage and Historic Scotland have very similar criteria for
evaluating any garden of historical or cultural interest, and whether
they should be registered.

I will summarise them here but they are best read in their entirety on
the sources provided below.
In general, the criteria and evaluation process is:

Age: 
Early gardens are obviously rare and will be registered. Later gardens
are more common and will be examined on date of creation and whether
original features remain intact.
Pre-1750 - any evidence of original layout
Pre-1820 - significant evidence of original layout
1820-1880 good or fair example of the best gardens of the period
1880 – 30 years ago, good examples of the best gardens
 
Then consideration of:

Influential sites, major designers, and good standard examples: 
The importance of the site; the association of the site with a
well-known designer; influence of the garden on garden design.

Associations:
Associations with historical events or famous people.

Group value:
Associations with other nearby sites, giving group value within the
local area.
 
Multi-phased sites: 
Gardens which have been changed over time will be considered as its
value lies in the series of developments it has undergone.
 
Documentation: 
Archives and published material add understanding of the site and can
contribute to its interest.

Plants and permanent elements :
Permanent elements in the landscape such as landform, built
structures, walks and rides, water features, structural shrubberies,
hedges and trees, and not at the ephemeral, shorter-lived plantings of
herbaceous perennials, annuals, roses, and most shrubs.

For more information on the criteria go to: 
http://www.english-heritage.org.uk/
On the top right hand corner click on Conserving Historic Places >
Designating Historical Importance > The Register of Parks and Gardens
> Criteria
(The site will not allow me to direct you straight to the page)

Calder Council have a good web page on its own gardens. There it
discusses the criteria in detail.
http://www.calderdale.gov.uk/build-plan/planning/regen/conserv/parks.html

In Scotland, Historic Scotland is responsible for the Inventory of
Gardens and Designed Landscapes .
http://www.historic-scotland.gov.uk/

You may wish to view a report which was compiled rejecting a garden.
It will give you an opportunity to see the assessment process in
action.
http://www.south-ayrshire.gov.uk/committees/publications/2002/PLAN0312/NEWTON.pdf

Other useful sources are:
http://www.gardenhistorysociety.org/ Garden History Society
http://www.nccpg.com/ National Council for the Conservation of Plants
& Gardens

Please ask for clarification of this research, or if the links do not
work, before rating the answer.

answerfinder-ga

Search strategy
gardens assessment “historical value” which lead me to the English
Heritage site.

"Register of Parks and Gardens" council
://www.google.com/search?q=%22Register+of+Parks+and+Gardens%22+council&hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&start=0&sa=N

I was a little disappointed in the findings as I had hoped to find a
little bit more information than is available from English Heritage,
the Government agency, and find out a little about some of the values
that EHs criteria have
formalised and the discourse about comparative values.  
regards