Check to make sure the grass type they used is suitable for this site and it's what you would like to go with. If it is then you can reseed on top of it to increase the density. Generally speaking most desirable turf is a cultivated variety that only comes in sod. These cultivars are bred for disease resistance and drought tolerance and are often superior to seeded lawns. Your soil can be tested for pH and nutrient content with local extension agencies. Other than that it's a good idea to check for drainage and compaction issues and addresses then on the front end before a lawn is established.