What is your definition of ornamental? Flowers? Foliage? Color? Most trees will overwhelm that space in short order. Would you consider shrubs, like New Zealand tea tree?

A small crape myrtle. A zuni or purple catawba type might work. Will be deciduous in winter but the bark is pretty. I have about the same size front lot and put in a catawba but is further from the garage eaves than yours would be. They are supposed to be drought tolerant. There are petite crape myrtles that get about 5 feet talls says the tag...which means 8 feet. Might be perfect for your size needed.

I've read crape myrtles can be prone to mildew near the coast, so check on the variety.

You might look into magnolias. There is one down the street I admire every spring and its in a raised bed about 5x5 and been there at least 10 years, its only about 8 feet tall and 3-4 feet wide, it flowers a pale pink.

try something like relatively fast growing, but not to tall like Dais cotinifolia,

Yes, to me pygmy date palms are quite decorative. So you should state what your idea of ornamental is.

Definitely something with decorative foliage or flowers. My short list is Metrosideros, Gold Medallion Tree, or Chitalpa. I already have manzanita Dr. Hurd on the other side of the driveway. Crepe myrtle would be a possibility, but I worry about mildew from the (May through July) June gloom. I've also considered a quince or an Angel Pink flowering cherry which I think would not be too large.

If it is three feet to the eaves of the garage I would not plant a tree at all. There must be nice shrubs that do not reach that height.

Zero supplemental water after year three will limit viable choices. Even once ever 3-4 weeks would make a difference. I'd suggest Heteromeles, but there would probably be funglal issues being that close to the coast.