If you don't need to renovate for your own needs, then don't. Since you don't know the time frame for the sale, there's an even chance that your renovation design choices will look outdated by the time you are ready to put the house on the market. You would be better served to start a savings/investment account to finance the remodel whenever it might occur in the future.

I agree with @CA Kate z9 and @Fun2BHere.

Renovate now if you need to do it for you. If you are happy with the way things are, don't worry about it.

I would emphasize planning to sell the home as is, rather than going through the stress of a remodel. You probably won't recoup the remodel costs.

Would your mother enjoy the renovation or hate it?

Homes that haven't been reno'd since 1960's make me salivate. No finishes that were installed recently that I don't care for but would hate to trash. So, unless you'd do this for you, don't do it for a future buyer.

A duplex (we called them that in Minneapolis) is a hot commodity. As everyone is saying, dont renovate for resale. Save your money (seriously, take that money and invest it for the future) and only do what makes sense for your enjoyment.

Paint.

Do not renovate for resale. Maintain the structure. Keep in good repair.

At this point, only make improvements for your own personal convenience, amusement or satisfaction. Do consider potential downsizing or relocation if it makes sense for you and your parent.

Just a suggestion but in some areas duplexes are being turned into a one family house to provide the space that is now desired. While I too would leave as is except for your own usage it could be a consideration if/when you decide to sell as a marketing tool.

Just another thing to consider since no one on here knows your state laws or if your mother has done any advance planning but in some states in order to settle her estate you may be forced to sell or keep the estate open for a long time unless each of you have enough money to purchase the other heir/s portion. If your mother's will mentions selling it and dividing the money some states treat wills as something to be followed or the executor must have a good legal reason not to.

Good point about an increasing number of multi-family homes being used by extended family. I was surprised to find a lot of these when we started shopping for a MFH as an investment property. In your case, your home is already used as 'extended family'. IDK exactly how positioning as one or another type of use might impact how you remodel, but I'd think you want to wait until the work will not disrupt your and your mother's lives. At that point you can decide if you want to continue living there, and/or if you want to increase the building's potential for rental income.

The property we're about to close on is currently used as duplexed-down on the first floor: 3BR 1Bath and Kitchen up, plus basement with 1BR, 1Bath and Kitchen. The second floor is 3BR 1Bath. We plan to make three separate units, adding a second BR in the basement, which has existing separate front and back entries and a separate thermostat. The second floor also has separate entries and thermostat. We may add laundry equipment in each unit, replacing the current shared facility in the basement.

I was amazed to learn how many MFUs exist in Chicago -- and how many are in great shape after over a century of use. I believe these will continue to be needed for a long time in a nation with a tremendous undersupply of SFHs.

Before you make a decision, would you be selling it from her estate or are you on the deed, or are you and your brother on the deed?

There are tax implications here that you all must consider.

I'd spend a few hundred dollars for a chat with an Elder Law Attorney.

Then there is the fact that Mom is 86 and in great shape...she could live to be 100. but, maybe not without help. Assisted living and in home health care are expensive. Does Mom have a long term healthcare policy? If not, could her savings cover $5,000.00 a month? Medicare only covers so many days (Medicare 100 day rule), and then it becomes a private pay issue.

Run out of money? The house can't be taken from her if it is HER primary residence, but after her death, Medicaid will recover what was paid out from the proceeds of the sale. Once upon a time, people could finagle the bagel and get away with moving money into different accounts to hide it from prying eyes. No more...I believe the look-back is 5 years from the date of applying for help.

It's complicated and I am not an attorney, but I have been in your shoes.

Long way around to say, do not spend a cent of your money until you get professional advice.

You, more than your brother (as he does not live in the house), have the most to lose as far as having a roof over your head. Maybe you should start thinking about buying a place of your own and your Mom get a tenant. Again, only a good attorney can point you in the right direction.

Renovation is messy and stressful. Don't put your mother through that. Don't forget, she's living with the memories of your father in that exact space. Don't disturb this.