This is something we most certainly did not agree to. Eventually everything was sorted out and we are very happy with our new home. But for nearly two weeks it was virtually impossible to lead a normal life. We effectively paid rent on a property in which we could only fully use one room. Moving house is a stressful enough experience without your new home being no more than a shell and having to live out of boxes for two weeks because the carpets are yet to be fitted.
New houses are great.
Just make sure they have actually been built before you move in. With hindsight, I would recommend that a separate contract is drawn up, detailing exactly what will be finished on the property and when. At least you have a point of reference and can claim compensation if the work is not completed. As regards lettings agents, they are always eager to get your money. Many people have experiences of lettings agents using tricks to get you put down your deposits and sign.