Well, if it is not on your credit report, it is not affecting your DTI at the moment. 3 things to consider though
1. if it will eventually affect your DTI and put you over the accepted limits for FHA (FHA has higher limits than most), you need to sreiously consider whether your are overspending according to a realistic budget.
2. Lenders can often re-pull your credit immediately prior to close of the house. This is even more true if the house is not yet built as it will take awhile and they have to have an accurate report at close. If you do not disclose the new loan and they re-pull credit and it has shown up, it can and probably will mess up the deal and possible cause you to loose some money as well.
3. They usually ask you to list your monthly debts at some point (at least both of the lenders I worked with did) technically, even if it is not on your credit score, not disclosing this is mortgage fraud. I am not sure it is in any way actionable, but it is wrong to a degree to not disclose the new account.
Personally, I would be honest with the LO and underwriter and if they have any issues witht he DTI, then the price is probably over what you should be at comfortably. To each their own on that, just be smart.