It sounds like you are in California. What part of California would be relevant. It also sounds like you only have room for shrubs. Unless you choose a tree with a fastigiate habit.

I think the Sweet Olive is a shrub that can be trained into a tree shape and there are probably others.

I have seen what looks like trees planted in some pretty narrow medians separating traffic in the road and in some pretty narrow grass strips on sidewalks.. So, I am sure there are small trees or tree-like shrubs that can work on my lawn.

This is in the foothills outside of Sacramento.

I don't think columnar evergreens will work. I think the city wants a more traditional umbrella/lollipop shape with wider foliage on top so that the most shade can be achieved from the smallest planter footprint

Smoke trees can be pruned as large shrubs or small trees. Star magnolias, some dogwoods and some Japanese maples also make good patio-sized trees. But I don't live in California so can't be sure those will work for your situation. Doesn't the city have a list of allowable trees for planting strips? Many do. If not, I'd make a trip to a local full-service nursery and talk to their staff. They know what works well in your area.

Yes, the trees I listed in the first post are ones I picked out form the list of pre-approved trees that also listed mature height in the 10-20ft range. Most of the other trees in their list were much larger.

They said they can also review and approve other trees. So, the trees in the list are not set in stone as the only options.

What about Crape Myrtle? They can be pruned to any shape and have very interesting bark and trunk patterns and need high heat to bloom. Columnar Hornbeam would also suit your application. Any tree will require regular pruning to fit your specifications.

Do some research on Japanese dogwood.

I have that here in Southside Virginia where we have very hot summers. The flowers are super bright white and packed closely together. Much better than our common Virginia dogwood.

As can be seen in the preceding photo Kousa dogwood has a very broad habit.

The planter area for the tree has a very narrow depth between the house and the curb of less than 10ft.

= a need for something narrow instead. With "fastigiate tree" not automatically equating to "columnar evergreen".

Going shopping at local outlets is a good idea because you'll be having to choose from what is on offer in your area at the time anyway. Unless you order your trees off the web.

Agree with the crape myrtle suggestion. They come in a ton of colors, different sizes and shapes, I'd look for a dwarf one, they love sun and they're pretty hard to kill and low maintenance.

Do plant nurseries generally sell crape myrtles that are already a few years old and that have already been pruned and trained into a tree so they can be planted with minimal maintenance?

I'm sure you could find a place that does but crape myrtles don't need a lot of pruning. Crape myrtles come in trees or bush ot just depends on the type it is.

Are there any disadvantages other than cost to buying crape myrtles or any other similar tree in the most mature sizes they sell? For instance, 15 or 25 gallon sizes instead of 5 gallon?

Are they more likely to get established and thrive in the new location if you buy small, younger plants?

To be honest I'm not sure, I've never gotten mature ones but a ton of shopping centers where I live have and they take.

I would just be sure to ask a local nursery that has one what soil they have it potted in, see if you can get one that's in similar soil to your home's.

I'd want something evergreen. How about one of the bottlebrushes? The birds and insects really do love them.

I'm not getting it, why the city can tell you what to plant on your property and suggest you take down what you have. What am I missing here?

And if you wanted ideas about it, I'm wondering why you would not provide a photo? You would get more relevant ideas.

What about dwarf fruit trees, they have flowers in the spring and fruits in the fall.

I have had a landscaper move a full grown crepe myrtle, size about 12 ft tall. It rooted immediately. A nearby house was going to be demolished and the crew said I could have the crepe myrtle, lol. I am in central NC, land of 100 degree hot humid summers.