The willingness of a seller to entertain this a logic for lowering the price of the home would depend strictly on the sellers level of motivation.
As mentioned earlier, the fact that no central sewer exists was incorporated in the original list price.
As you state, you wouldn't be able to put a pool back there or potentially expand. Other than that, the value of the lot is measured by it's condition, privacy, space etc. has Hal has stated. Some people want pools, most don't.
If you are still looking in Potomac, then septic fields are common. However, most (not all) on septic in 20854 are on larger lots of ~2 acres.
If this is an older home, built before such expansion space was required, it may prove to be a valid concern for the purchaser due to the fact that they may have to pursue a potentially expensive alternative to typical septic system as the system ages. They may also consider purchasing easements for adjacent lot owners for expansion.
If however this area is simply located in the center of an otherwise typical lot, 3/4 acre or more, then it should not affect the market value for the majority of buyers.
It's necessary to consider the whole picture to advise you properly. I am happy to help!