I have a slightly different take on this. We have a new build bungalow in a county town in Scotland. It's four years old and we're selling it now for 20% more than we bought it for because the developer has a new site in the same town to build another 200 houses, but not for another two years. We're getting out while we can because we anticipate difficulty selling when similar new homes go up.
On the other hand, 10 miles down the road in a similar sized town, there are people selling four year old houses for 20% less than they paid because they paid far too much to the developer. The estate still isn't finished and we can't afford to buy on it.
I think you need to know the size and specifications of the new houses, what the price will be and see how it compares to what you're thinking of buying. It may be that there's no comparison and what you're buying won't in all honesty be compromised because it's bigger, more private, has more garden, basically the same EPC, number of bathrooms and will still be cheaper than the new builds and be saleable with a profit for you if you need to move. If that's the case and you like everything else about it and can cope with the road, go for it but only offer what you feel it's worth to you and be prepared to move on if you can't get it.
We are going to a very large, old house on a main road, rural with views of fields. It has more rooms, more space and more privacy than an equivalently priced new build, but it's been cared for. Compromise will be road noise to get used to and limited parking. Not planning ever to move again, but if we did, we're looking to pay £30k less than it's valuation because they've had it on more than a year and dropped the price.