It might be useful to look at this (a charity): https://livingmadeeasy.org.uk/
As well as what other posters have said:
Properly non-slip flooring throughout. No rugs or other trip hazards. If having a patio outside that should be non-slip, too- not slippy wooden decking. Perhaps covered porch/awning over part of patio so that older person can get from car/scooter to front door, and find their keys etc in the dry. Previous poster's suggestion of box for deliveries = very good idea.
Ideally, bathroom/loo to be easily accessible from bedroom, for night-time visits.
PP's suggestion of laundry area= another good idea.
Window fixings as well as door handles and tap handles that old person can manage.
Consider old person's attitude to/aptitude for technology. There are all sorts of clever remote controls for all sorts of devices, but some are small and fiddly. I know a 90+ person who can manage tech-controlled devices really, really well, but also a 70+ who has very little patience with them
Building Regulations now also have various requirements for all new and some converted buildings - though NOT extensions - to accomodate people with limited mobility : https://www.gov.uk/government/publications/access-to-and-use-of-buildings-approved-document-m
This advice note from the National House Building Council lists all the topics that are covered by accessibility Building Regs, although many/most of them won't apply to a private dwelling. Your architect will be able to advise.
https://www.newbuildinspections.com/knowledgebase/nhbc-part-m-disabled-access-regulations-uk/