If you plant a tree in a lawn, you need to leave a circle of a meter ( at least ) in diameter around the trunk with no grass - cover it with chipped bark or another mulch if you like . Because
- your baby tree can't compete with grass forfood and water
- the roots are mostly near the surface and vulnerable to damage
- the trunk is less likely to be damaged when you cut the grass
For a feature tree, I'd want at least two seasons of interest . Eg flowers or pretty foliage in the spring and good autumn colour /berries
If you live in a city , you might need a tree that tolerates pollution
Evergree conifers cast a lot of shade , so you need to be careful
If you go for deciduous , remember it will be bare for quite a large part of the year, so think about the form ( what the tree looks like naked ) eg cherries are very pretty for two weeks of the year but can be lacking interest for the other 50 ( unless you get one with great bark)
Some trees have a very dense canopy and cast a lot of shade , others less so
I have a very exposed garden ,so I've planted very tough trees eg sorbus, betula ( birch ) , malus ( crab apple ) , crataegus ( hawthorn ) , yew, holly , etc . In a city you have a lot more choice .
Remeber that a " very small " tree is 5-10 m . A small tree might be 10-15 m. Check that you actually know what a 10 m tree woudl look like in the position want it . Check that you have enough width for the tree you want
L ots of very small town gardens would be better off with a large shrub