First - Tax Assessment has NOTHING to do with home values and you can never compare them.
Tax Assessment is the value that your local county / city / town places on all homes within their jurisdiction to assess how much each owner is required to pay for property taxes. Property taxes is used to pay for public schools, police, fire department, public parks, city / county employees, sanitation, maintenance costs, public projects, health care benefits for employees, retirement benefits for employees, etc.
Market Value is the value that buyers will pay for your home. Value is based on similar type homes to yours that have recently been sold (within the past 3 - 6 months), homes that are currently under contract and homes that are currently for sale. Market value is dependent on the local economy, employment situation, neighborhood information (crime, shops, restaurants, cleanliness, wealth, etc.), quality of local public schools, access to public transportation, convenience of local highways, distance to nearby cities, commute to jobs, etc.
With new construction, the tax assessment will always be 0 until the city / county comes to assess the property - after it's built. Builders need to sell the new home quickly so the new tax bill is paid by you - not the builder.
The other issue that concerns you is builders charge a higher premium price for new built homes (new construction) because they can easily get away with it. Builders know that buyers like you LOVE brand new construction homes so if you had refused to pay that "premium" price the builders were selling it for, it would take them anywhere from 5 minutes - 1 day for the builders to get a new buyer to pay their premium high price. Builders don't give a rat's ass how you feel about paying their higher premium price. If you disagree with it, they can easily tell you - "Sorry, but I refuse to negotiate. Good luck to you finding a new home. I'll just wait until a new buyer comes along in 10 minutes." This is how builders get you.
What you need to know is it will take you about 4 - 5 years for you to gain any equity on your "new" home. Once you move in, your home is NOT considered "new" anymore. If you sell within the next 3 - 5 years, you will lose money because your home will be compared to older homes instead of the new construction homes. This is why you need to live in your home for at least 5 years otherwise you will need to bring cash to close on your home. By the way, builders, realtors and lenders know this but it's a secret they never tell buyers. Otherwise, nobody would ever buy new construction homes.