Several factors could have caused this hard layer of clay.  In a new construction area, the top soil is almost always graded off leaving a hard, very dense, compacted layer that water and air could not penetrate.  When planting a new lawn a layer of top soil was probably added and built up over years .  If you want to plant bushes, perennials, etc., you would be wise to open up and amend the planting area so that water could penetrate and roots could thrive.  Most plant problems are caused by soil problems so it would be to your benefit to correct the soil as much as possible. 

I would recommend double digging the bed.  Double-digging adds air deep into the soil and enables roots to grow and the microbes to create good soil structure.
How to double-dig: Dig a trench 1 foot wide and 1 foot deep and put the soil in a wheelbarrow or on a tarp. Work a garden fork into the bottom of the trench and rock it back and forth to loosen the soil. Dig a second trench and put that soil in the first trench. Continue until you reach the end of the bed. Fill the last trench with the soil excavated from the first.

Additional important information about compacted soil can be found here:
http://www.ext.colostate.edu/mg/gardennotes/215.html

Good luck.