Elevations and plans showing orientation would help. Additionally, how do you typically use your deck? Do you spend a lot of time on the deck or do you just go out occasionally to grill and then come back in?

build her a screened porch under the deck - with some nice lights etc

We need some idea of the house you are building . IMO there are ways to handle this issue in the layout of the house but we need more info.

Decks located above windows will inhibit light from entering the space those windows are in. Design the house so the deck is not located where it will inhibit light from entering the space in the level below. Design the lover level to accommodate how the space will be used over the next fifty years.

The slope of the property will have a direct impact on the light and the positioning of her apartment. If the entire house is raised so that the basement has windows all around, then place her apartment so that your deck overhangs spaces that have no windows: closets, bathrooms.

A secondary living space designed for an older occupant needs way more thought about whole property accessibility than it does about sunlight. And Sunlight is important. It’s just less important than the whole. It’s part of a whole.

Are you putting in an elevator? How will she access her basement apartment otherwise? Stairs? Really? If she still drives, where will she park? Bring in groceries? What is the route from entering the property to getting to her space? Is she expected to never leave that space and be with the family upstairs? How will that happen?

You need better design help than you’re currently getting if you’re just concerned about a deck for the upper floor occupants.

We have a basement suite, not for an older person. The suite is side entrance though as our deck would have made the area far too dark to enter from. As a result the suite is actually a one bedroom and den and the den has no window.

If I were to build again I would rethink the suite location and make it more accessible so my in laws could have lived in it. That said a basement suite is extremely marketable in my area so it’s been fine.

I would never put in an elevator and have never seen one in a home near me ever. I’m assuming you are putting lots of thought into your design and hope you find a way to make it work.

We are currently finishing our walkout basement. The upper floor extends over the basement by about 14’ and the walkout faces north. That said, I have been very pleasantly surprised by how much light it gets. With the natural light and extra care with the lighting and paint, it feels nice and airy. I’ll try to get a picture tomorrow.

Instead of a deck, use stairs down to a patio. Or, at least, a low level deck.

To provide maximum light, abjure window wells. IOW, lift the first floor, so the basement is partially above grade. If you're still stuck with window wells, consider large well surrounds. Or even a walk-out arrangement such as the one below.