I doubt much remediation is necessary. If just old plants/plant trimmings and a few miscellaneous odds and ends, there is minimal concern about any chemical contamination. But you can have your soil tested for that if you wish. Just clear all the junk away.

There is a much greater concern regarding the existing native soil being removed and/or compacted during the construction process but bringing in new, enriched soil before any planting and mixing it in a bit will alleviate that issue. Then just plant as you wish - lawn seed or sod will not make any difference.

If it's just trash I wouldn't worry about it too much. As an env. chemist I can tell you that average household trash is not going to pollute the soil significantly. Parked vehicles don't pollute much unless there is a big oil leak but that's fairly rare. The main risks of vehicle maintenance, junkyards etc. are petroleum products and lead from batteries. But unless he was doing maintenance there AND very poor housekeeping, it's not going to be a concern. As far as landscaping work, if he was spraying chemicals he might have mixed them there and created some spills. That would be hard to detect by looking. Sampling is possible though expensive for pesticides and you would have to know what you are looking for (i.e. the exact pesticides that were spilled). As an env. specialist I once worked on a site where a guy had a termite business and mixed sprays in a tank on his pickup truck right in front of his garage, on a gravel drive. There were pretty high levels of pesticides in that small area. A truckload of soil went to the landfill and all was well. The problem is knowing where to look, and for what.

If you are picking up trash and you see batteries, oil containers, or pesticide/herbicide bottles, it's possible there were spills and resulting contamination.

I'm giving you the worst case based on what you described. It's entirely possible there is nothing going on here to worry about. Did you talk with the previous owner? How confident are you that it was only household trash? You could ask if he mixed any sprays there, and tell him you only want to know in case there is something like Roundup that could affect your new lawn. He's likely to be more honest if he doesn't think you're making a big deal out of it.

Any RoundUp spills won't affect your new lawn.......not unless it was spilled just last week :-) RoundUp breaks down in the soil quite rapidly and one can replant anything in the area in just a couple of weeks or so.......certainly well before the construction on your new home is completed and landscaping can begin.

I used that as a fairly innocuous example that s/he could use to put the former owner at ease. If he was actually mixing pesticides there he may have used more persistent stuff than that. Kinda doubt he did, if it was a vacant lot there wouldn't be any water for one thing.

By the way is the builder removing soil around an existing house for the purpose of cleanup or to get a more stable foundation for a new house?