If you're working with a builder or an architect. They should follow 'The Principals of Universal Design' which provides detailed requirements for building with the disabled and senior citizen independence.

Check this site out. It will be extremely useful whether you're from Canada or the US.

http://www.cmhc-schl.gc.ca/en/co/acho/acho_001.cfm

No tub. My mom is 84 and can use walk in shower only or if you prefer do a walk in tub/shower but nothing you have to step into. Laundry on main floor. No steps. Phone jack in living room where they sit the most. No high cupboards. My mom is to weak to lift anything down from a cabinet. I'm sure I could think of more but these are a few main ones.

Grab bars in shower as well. We had to install them in my folks' condo after my mom had surgery, but would've been easier to have them there already. If they will be doing own laundry, consider what will be easiest to access - front loader vs top loader, and if front, consider pedestal so not too low to ground. One floor living. Careful of tripping hazards like throw rugs or changing floor materials with thresholds. Also consider some kind of security system that includes an option for calling for help. My folks just moved from their condo to an independent living /continuing care community so their new space is designed for seniors, but I haven't seen the finished product yet so cannot comment on what else they have.

92 year old Mom. Here is my experience compacted down. As mentioned, tripping hazards. Very important. Lower beds. Hand rails by toilet also. Preferably a bathroom sink that one can roll a wheelchair up to and still reach faucet. Shower large enough for walker type seat. Hand held shower head to reach all areas they can't. A comfy chair they can easily get in and out of. Preferably one that rises to a standing position. The bathroom that will be used the most should be tiled floor and halfway up wall. The ugly truth is there will be accidents! I'm sure I will think of more.

Interior doors large enough for wheelchair access. Hand rails even if it's one step up to front door. Not sure if the condition your parents will be in by the time they come live with you, but my mom has very bad knees and needs to hold onto something even if it's one step up.

Don't put their toilet in a separate toilet closet. It will be to narrow for someone to help them when that time comes or they might need a walker to get up to it.

No outside steps.

Security lights for exterior- motion sensors and timers.

Patio designed for wheelchairs, walkers, and canes.

All entrances designed for all-weather access. Decks get slick in rain and turn into ice rinks in snow. Heated sidewalks could be an option, depending on your climate.

garage with room for fully opened car door + helper+ wheelchair. Cars can be a challenge and you want to enter and exit in a sheltered area.

Walk/roll-in shower with adjust wand sprayers and stylish grab bars.

Grab bars can be more arty function than just medical function and are also for the able-bodied.

Kitchen counters with varying heights from wheelchair up to standing.

Easy access to kitchen faucet controls.

Easy access to oven- in chair, walker, or standing.

Easy access to stove top seated or standing.

Easy access to microwave seated or standing. For some, it is safer to allow microwave access than stove top.

Electrical outlets raised up from standard wall height. (away from toddlers and closer to wheel chairs.)

Light switches lowered closer to toddlers and wheelchairs.

No knob door handles.

All door ways wide enough for wheelchairs+ a bit. All rooms planned for wheelchairs to turn around in plus furniture space- not galley bathrooms.

Closets functional for wheelchairs and standing folks with easy reach or pull and lower shelves.

Pantries can be designed with pull out and down shelves.....

More light from more fixtures. As eyes age, they catch less light and cataracts make that worse. Higher output LED bulbs can make a big difference. Lighter or more reflective paint surfaces help bounce light around. Glass or shiny metal accessories reflect even more.

Night lights and safety lights for finding the way to kitchen and bathroom are thoughtful.

Lights marking emergency exits are wise. Maybe the led tape light strips along baseboards?

Timers for some lamps in darker interior rooms to remind of daylight and sleep cycles.

Dimmers for bedside lamps to prevent falls while in bed. They are made for specific bulb types: halogen/incandescent or CFL/LED. Plug dimmer cord into wall and lamp into cord and leave dimmer control on bed side tables or on wall by bed. Use dimmable bulbs. I also like using the 3-way sockets with bulbs for high-use area lamps. You have accent, ambiance, and reading light from one lamp and helpful for the darker seasons.

Hearing also may deteriorate, so plan for headphones/pillow speakers etc for tv watching all can enjoy. Sound insulating the interior walls is no bad thing, either. It's not just teens cranking the tv or radio to apocalyptic volumes.

Doorbells are not audible throughout the house- sound+video doorbell could be an option. Flashing light doorbells as for the deaf/hearing impaired are another idea.

Smoke/fire/gases alarms- should also be flashing something and not just all sound.

There are a lot of on line articles that will help, as you don't want to miss anything important. The article below discusses 7 principles and then subdivides with specifics.

https://www.caring.com/articles/7-design-secrets-for-a-house-you-can-live-in-forever

If there are vision issues it is a good idea for the floor color to change from room to room so they may stay orientated, but as mentioned earlier- with minimum transition heights.

Switches for lighting, etc. amongst kitchen cabinetry may be placed inside a cabinet door camouflaged as a drawer front for easier access.

Elaborating on earlier post- a microwave drawer mounted in a base cabinet may be a good option. The controls may tip down for easy reading, the drawer is powered so there are no spills, and it is easier to access the food/drink without having to reach back inside the microwave.

When building new and wanting to have more than one level, some choose to have an elevator shaft installed with the option of adding the rest of it later, using the space as a butler's pantry in the meantime.

These ideas are fabulous –
especially those that pertain to slippery areas within the home. Water prone
areas such as the kitchen, entryway, garage, patio/porch, and (most
importantly) bathrooms are often the scenes for slip-and-fall accidents. There
are DIY treatments homeowners can apply to either etch or coat any floor
surface, adding additional friction for walking about the home.

This is great for seniors who are
joining the smaller house movement. Many older adults are downsizing to smaller
spaces, which may allow them to finally enjoy the flooring of their dreams, but
with safety in mind.
There are even DIY coatings that can be applied
to ramps, ramp railings, bathroom grab bars, or any other spot in home where
extra grip is needed for older individuals.