Habinteg is a provider of accessible homes in the UK. They have developed a Web-based Toolkit for Planning Policy that includes accessible housing. There are several tabs including one on the cost benefit arguments.
A review of Part M of the UK building code was commissioned to see what the costs would be to upgrade Part M of the building code. It was calculated at an additional £521, which is about 0.2% of a new house. Habinteg claims that no attempt was made to weigh any additional development costs against cost savings in other areas. For example, avoidable hospital admissions due to falls, impact on social care costs, and long stays in hospital due to no suitable home to return to. The calculated additional £521 for improved access standards would be more than met by avoiding one week in residential care.
You can download Habinteg’s response to the review from the Toolkit webpage. The website also has some informative videos.