From the description you have provided, it looks like that in one single compound you have 2 separate houses?
Is that correct? Both of these buildings are for 1 family or multiple families are going to be there?
According to me, new house should be treated as a new building and decide entrance for that new building only. Don't consider old building part of this new building. If possible, you can also have a soft boundary between these two buildings.
In your case you should use the maximum positive zone gate entrance.It will give you good benefits.
my house facing east direction and main gate is over the under ground water tank . is it right or wrong ... what should i do now
In a flat/apartment sunrays do not come from main entrance, it come from front or balcony of flat.So in flat/apartment is main entrance in N/E or E necessary or the front/balcony should be considerd in N/E or E direction as the sun rays come in whole flat from front/balcony.
Sir my house is facing north east 40 degree and somebody told that we can apply same vasthu as house facing north exactly, if house is between north and north-east is that true? Thank you sir.