Well , this is really a very good question , and it seems you know Arabic very well :)
But first of all : ( الجميل ) means : ( the beautiful ) , not : ( the new ) .....as my partners said :)
So , there are two ways to say this :
1- You can simply say , as you said , : ( باب البيتِ الجديدِ ) , so : the adjective : ( الجديدِ ) refers to : ( البيتِ ) .
2- Or : if you want to say : both of them are new , then you can say : ( ِالباب الجديدُ , للبيت الجديد ) .
And you are very welcome :)