Dad said yes to a stair lift. Good. That is permission to measure the job, not permission to let a salesperson choose the job.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
A stair lift can fit the length of the rail and still fail the house. The parked chair can block a doorway. Dad's knees can meet the opposite wall. The top stop can leave him standing beside the first descending step. A folded seat can turn the only staircase into a sideways squeeze for everyone else.
The useful question is not, "Will your model fit these stairs?" It is, "Show us where Dad's feet, knees, hips, hands, and next safe step will be at both ends."
Measure the staircase while it is still being used
Start with the narrowest clear width, not the widest tread. Measure from the wall or handrail on one side to the nearest obstruction on the other. Repeat at the bottom, middle, turn, and top. Write down anything that steals space: trim, radiator, window ledge, handrail brackets, newel post, door handle, or a wall that closes in near the landing.
Then photograph the staircase from both landings. Take separate pictures with every nearby door fully open. A basement door, coat closet, bathroom door, or front door can cross the proposed rail even when it looks harmless while shut.
Mark where the folded rail, chair, arms, and footrest would sit. Ask the installer for the folded projection of the complete unit, not just the seat. Accessibility and installation guidance treats landing clearance, obstructions, folding sections, and an available route past the parked equipment as separate issues. The documents behind those checks are collected on the Sources page.
Now have the person with the broadest shoulders in the regular household walk the stairs carrying a laundry basket. If that route matters during an emergency, ask the installer to state the remaining clear width in writing. "Plenty of room" is not a measurement.
Fit Dad before fitting the rail
Dad should test the proposed seat or a demonstrator with the coat and shoes he normally wears. Check these positions:
His hips reach the back of the seat without being shoved or twisted. His feet rest fully on the footrest. His knees clear the opposite wall, handrail, and any passing trim. He can lower and raise the armrests without trapping a hand. He can operate the control with the hand he expects to use. He can fasten and release any restraint himself, if that is part of the agreed setup.
Seat height matters at both ends. Ask the installer to show the finished seat height above each landing. Dad should be able to sit and stand from that height without planting a foot on a stair edge. If the proposed height is based only on a tape measure, stop. A chair that fits his lower leg on paper may still put him too low to rise or too high to sit under control.
Do not quietly promise that a family member will lift, catch, or rotate him every time. If the arrangement depends on another person, write that person and task into the plan. Otherwise the house has acquired equipment that works only when the right witness is present.
Make the top landing prove itself
The top is where a tidy rail drawing can become a bad transfer.
Ask Dad to point to the exact place where he expects to stand after leaving the chair. Check whether both feet can reach a level surface before he lets go of the armrest. Check what he can hold after the chair swivels. Check whether a bedroom, bathroom, or closet door enters that standing space.
Ask whether the seat locks in the boarding position and whether the lift can move while the seat is turned. Ask the installer to demonstrate the answers, then put the promised configuration on the drawing or proposal.
If Dad would step directly toward the stair opening, ask what changes the geometry. Possibilities may include a different rail side, a powered or manual swivel, an overrun arrangement, or a different parking point. Those choices are not interchangeable. Each changes the landing space, the controls, and what Dad must do.
Make the bottom landing pass the door test
At the bottom, tape the proposed end of the rail on the floor. Open every nearby door. Walk the normal route to the kitchen, bathroom, basement, and outside door. Look for a rail end that becomes a shin-level obstacle or prevents a door from opening fully.
A folding rail may solve a doorway conflict, but it creates another job. Ask who folds it, whether Dad can operate it from where he stands, and whether the lift is prevented from running when the section is out of position. Ask what happens if the folding mechanism loses power. Guidance for stair lifts specifically treats folded rails, boarding points, obstructions, and emergency operation as matters that must work together, not optional polish. See the supporting material on the Sources page.
Choose the rail side by transfers, not convenience
The shortest installation is not automatically the usable installation. Compare both sides of a straight staircase before accepting one.
For each side, check Dad's stronger hand, the direction he turns to sit, where he can park a cane, whether the upper handrail remains reachable, the door swings at both ends, and the route from the chair to the next stable surface. On a curved stair, ask for a drawing showing every parking and boarding position. "Custom fit" should produce a custom drawing.
If another person regularly uses the stairs, test that route too. Dad did not agree to make everyone climb around machinery. Keeping the remaining handrail usable may matter more than hiding the rail against a preferred wall.
Ask what happens when it stops halfway
Battery operation does not end the power question. Ask which household outlet supplies the charger, whether that outlet must remain switched on, what indicator shows charging, and what Dad should do after an outage.
Then ask the harder question: if the chair stops between floors, how does Dad call someone, and how does an authorized person lower or move the unit? Ask for the exact procedure in the manual. Do not accept "call service" as the complete answer when Dad may be sitting over the stairs.
Identify the electrical disconnect, manual movement procedure, and service number. Put those instructions where a family member can find them without searching through a drawer of warranties.
Keep the authority with Dad
If Dad owns the house and understands the choice, the decision is his. Agreement to an estimate is not agreement to drilling. Have him approve the rail side, parking positions, control hand, and who may receive service information.
If the house is rented, part of a condominium association, or shared with another owner, check the lease, association rules, and title before installation. Ask the installer who is responsible for permission, permits, electrical work, wall or tread damage, and removal. Local requirements can vary by municipality and by the work actually proposed. The Local Help in St. Louis page is the place to start locating the correct local office, not a substitute for asking that office about this address and scope.
Make the proposal describe the installed house
Before Dad signs, the proposal should identify the exact staircase, rail side, seat and footrest configuration, upper and lower stopping positions, folded projection, remaining stair width, power source, door conflicts, and any folding or swiveling parts. It should also say who performs electrical work, who handles required approvals, what training occurs at handoff, and what happens to holes and wiring if the unit is removed.
Require a final demonstration with Dad. He should call the chair from both ends, board, operate it, leave it, fold it, and use the stairs as far as he normally does. Test nearby doors with the unit parked. Confirm that another person can still travel the staircase.
Your next move tonight: take six photographs, measure the narrowest stair width, and put painter's tape where Dad's feet must land at the top and bottom. Send those items to every installer with one instruction: quote the same route and show the parked chair, rail end, door swings, and remaining walkable stair in the drawing.