Clay is a term that applies to a generally heavy sticky soil which is difficult to work.  Sticky and heavy when wet, hard and lumpy when dry.  Commonly soils used on new house construction have had their mechanical structure destroyed through the handling and storage process, particularly on green-field sites.

While soil structure may not be good it does not make it a clay soil but nonetheless many of the problems to be faced are the same.  Commonly the soil structure is such that drainage will not be effective, especially during periods of very wet weather and the winter, resulting in a sticky slippery lawn which would lead to encouraging worms and weeds.  Correspondingly in the summer dry periods the soil may crack and be very hard.

When creating a new lawn on this type of soil attention should be given to improving the existing soil structure.

This can be achieved by placing a simple drainage system into the soil and bringing the gravel up to the surface before capping the whole area with a suitable light top soil, ash, or turfing sand to a depth of at least 50mm.  To incorporate sand into the soil itself in a volume that would make a fundamental difference iwould require 80% to 90% of the resulting material to be sand.  This is a very big task and if you are faced with no alternative but to work with the soil you have, then capping is the best way forward and recognising that in the longer term you may have to carry out regular aeration work.

On small lawns it is probably most cost effective simply to have the offending material removed and replaced with good quality top soil.  There are a number of soil conditioners on the market which can assist the breakdown of clay soils.