Hi Helen,
When we ran the housing standards seminars last autumn it was really hard to find any LPAs that had done very much work on collecting the evidence to justify adoption of the optional standards in their local plan. The exception was the GLA's minor alterations to the London Plan. This has now gone through examination and all the reports, including the inspector's can be found at https://www.london.gov.uk/what-we-do/planning/london-plan/minor-alterations-london-plan/minor-alterations-london-plan-2015#Stub-123267
Different evidence will be appropriate depending on the standard that you are looking to impose. The first question for all however needs to be whether the existing market is actually delivering to an acceptable standard. Housing colleagues will undoubtedly be helpful, as will the EA in terms of water shortages.
You might like to look over the slides from the seminar series. http://www.pas.gov.uk/web/pas1/planning-positively-through-partnerships/-/journal_content/56/332612/7347558/ARTICLE
In terms of the accessibility standards, one of the most telling messages from the series was not so much about evidence gathering of need - but about the effect of the standards on deliverability. The building regs dont have the flexibility that we are used to having through DM standards. So its important to understand how the standards will impact on design and layout and therefore deliverability - particularly on sites in constrained town centre sites and on sloping sites. Best advice is to talk it over with your building regs colleagues and avoid policies which require a blanket ratio of high accessibility homes across the whole district.
Phillipa
