Rudbeckia'Early Bird' flowering now,is a nice one...about the height you specify..I'm sure other members will have lots of ideas for you...

Some types of crocosmia are wonderful - emily macKenzie is a beautiful yellow

Rock roses come in yellow shades, stay low and flower a long time. There are yellow asiatic lilies about that height. Some herbaceous potentillas stay short but be careful as some grow tall and flop over.

thanks for the recommendations. will look up the names you have mentioned to see what I like. thanks again - you've given me a starting point

These are some of the shorter yellow perennials I grow and I find I have yellow colour from spring to late autumn.
Helenium autumn premium magnificum is under 2ft, Primula veris (Cowslip), Primula bulleyana, Rudbeckia fulgida 'Goldstrum Coneflower, Solidago Golden Dwarf,
Allium Moly, Bidens Aurea Hannay's Lemon drop, Bidens Bur marigold, Coreopsis Grandiflora heliot, Coreopsis Grandiflora heliot, Coreopsis verticillata 'Zagreb' Erysimum Fragrant Sunshine, Erysimum' Golden Jubilee'. Some lovely yellow Sedums too.
You can also add bulbs such as Narcissus, Crocus and Tulips also Iris and annuals like Pansies to give you yellow all season.
I have found that yellow does not mix with other some other strong colours so well and have now made a large bed of just yellows/white but that is a personal preference!

wow ! many thanks to drc726 - will look up the plants mentioned to chose something to my taste. really appreciate your input - thanks !

Your welcome