The spare room looks like the answer. It has a door, a bed, a window and a closet. But a room is only a plan if the rest of the house works around it, and the questions that matter are about the route, the stairs and the hours, not the square footage.
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.
Walk the night route
Stand in the bedroom and walk to the bathroom with the lights off. Count the steps, the doorways and the corners. Check whether the floor is level, whether there is a rug that slides, and whether the light switch is in reach from the bed. If there is a stair between the bedroom and the bathroom, that is a different house than one where everything sits on one floor. Write down the route in plain steps so the family can see it.
Check the doorways and the turns
Measure the doorway from the bedroom to the hall and from the hall to the bathroom. Some parents use a walker or a wheelchair at some point, and a doorway that is tight for a person can be impossible for a chair. You do not need to buy equipment to measure. You need the numbers so you know what fits before a chair is delivered to the wrong room.
Count the stairs honestly
Count every stair between the bedroom, the kitchen, the front door and the car. Note which ones have a rail on each side and which have a rail on only one side. Ask whether your parent can manage those stairs alone on a good day and on a bad day. If the answer is no, the spare room is a room on another floor and the plan has to say who carries what up and down.
Name the hours you are actually giving
Write down your real weekly schedule, including the shift that starts early, the kids who need a ride, and the job that sends you out of town. Then write the hours your parent will need. Appointments, meals, medicine reminders, bathing help if it is needed. Compare the two lists. Where they do not match, that gap is the part someone else has to cover, and it is the part most families discover in week three.
Set a trial length and an exit
Agree on a first period, such as a month, and a time to review it with the siblings. Write down what would make you say the arrangement is not working. Keep a line for the option of moving your parent to a different setting if the route, the stairs or the hours stop fitting. Families that set an exit at the start are less likely to fight about it later.
Find help that fits your home
If the route checks out but the hours do not, look at the local options before you decide. The local help page on this site lists the kinds of support available in the St. Louis area, and the sources page shows where the information comes from. Use both to compare what you can cover yourself and what needs a second person.