Thanks mydreamhome! That information is tremendously valuable. I just talked to a sanitiarian at the local Environmental Health Dept. He said exactly the same thing as you, that the whole house filter may be the best solution but expensive. I just realized the nitrate level is not just a little high, it is over the roof. It is currently 236 mg/L as N versus the standard MCL 10 ppm. After the RO system is put in, it's reduced to less than 5 mg/L as N. I don't know exactly what these numbers mean esp. because they seem to use different unit terms (mg/L as N vs. MCL). But the sanitarian said 236 is alarmingly high. I would not want to bath my baby and wash vegetable in that water.
On the other hand, I'm certainly not the only one having the problem in that subdivision. They've seen other cases. This piece of land was converted from farm land and there are also farms nearby. I don't know if it's more fertilizer or animal waste that's contaminating the water.
I am going to talk to the builder and ask them to pay for the installation and maintenance of a whole house RO. But I don't know if I should discuss the additional energy and wasted water that will turn into higher electricity and sewage bills. Are there some perameters here you can share?

oops, I have to revise the message above. The county said it is not 236, but 23.6. That makes me feel much better. Now I have to consider whether it's worth the fight to get the builder to do whole house.

Why is that nitrate level harmful for bathing? Nitrates absorbed through the skin? Unless the well was drilled AND tested before the contract of sale was executed, the builder had no way at all to know the water quality.

Ammonium nitrate is a commonly used fertilizer.

Here is a link that might be useful: Ammonium nitrate

Ultimately the choice is up to you about whether to opt for the whole house system. Two of the houses that we have owned had to have RO systems due to nitrates. High nitrate levels are fairly common in my area as it is mostly rural farm land. Nitrate levels can vary throughout the year and we felt it was the best option to go ahead and put the RO system in. Installing a whole house RO system should not be too much of an issue for the builder, just approach them and discuss your concern with them. If you do not feel like asking for it you can also purchase the complete systems at building supply stores such as Home Depot or plumbing supply stores. If you are relatively handy it is not too difficult to install. More than likely they were not aware of the nitrate issue until the water sample was sent out for testing near the completion of the home.

Better research more. The filters on RO systems are expensive and require changing at least once a year. Under sink systems generally last 6 months. We were told high nitrates can cause 'blue baby syndrome' but are otherwise not harmful. That was 20 years ago.

It is related to animal waste and generally found around farmlands. I would not buy a house with this condition. I lived with it for a long time with RO under the sink. It was expensive as were the water tests which had to be done regularly. I used RO water for all cooking and drinking. I would not use the ice machine because it was not hooked up to RO. I was also told inhaling the steam from the shower was harmful.

We were told to dig a deeper well to tap clean water. We did not because of cost. Could you get them to dig a deeper well?

This fact sheet seems reliable.

Here is a link that might be useful: Nitrates