Wasn't it Erma Bomback that wrote "The grass is always greener over the septic tank"? Sorry, I know that's not the answer you were seeking but I couldn't resist.

FWIW, I don't think your proposed deck extension would be over your drain field. If it were then your current deck would be over it as well.

There may be a pointer or two from within the house. I was faced with similar dilemma years ago and spent a lot of time probing behind my home - to no avail. Turns out it was in front of my home. @#$%^

One indicator may be to see where your main sewage line leaves the house... if it exits through a foundation wall, that tells you it likely heads out directly toward the septic tank.

Secondly, if when the home was built if the builder had knowledge as where your future sewer main might be routed, he should have located the tank and drain in the general direction of that future sewer line - if possible.

My son has a home that the future sewer line turns out to be planned to run behind his property. Sure enough, he had been looking in the front.

Haha, I think if the grass is greener it could be a sign of a problem though? I think you are right that the deck will probably fine, but it sure would be nice to know where it is for other reasons.

Stax, I do know where the tank is- hopefully most home owners do since you definitely don't want to cover that up! (routine pumping)

Gravel should be 6 " more or less down. Keep in mind a typical leach field is a series of pipe runs in trenches so only a portion will contain gravel. There's no way there's leach field under your proposed deck.

how far to the right does your lot go? it may be that open spot in the woods. Or at the upper part of the picture toward the road.

Even though I own everything shown, technically the lot the house is on only goes to where you can see a power line running across, basically in line with the pine trees running north and south. The open area is another lot that could be sold. I have no idea how big a drainfield typically is. Part of it is definitely at the yellow dot in the picture, because that's where the inspector augered a hole to inspect the gravel. I wasn't sure if it would spread out farther once it got past the deck, or past the house going north.

I have this same concern. We want to have in ground sprinklers installed, and the installer is concerned about bumping into the tank. I forgot to mark the access hatch the last time the tank was emptied, but I can find it again, I'm sure I know where it is. However, how do I determine where the tank is from that point? If the cleanout pipe (or whatever it's called), is exposed, seems to me the tank and leeching field could be in any direction from there. It's not marked on the survey. I asked the town if they have records, but haven't heard back yet.

How long have you lived there? Sometimes the leach field lines show up in the lawn as dried grass strips in dry summers. Or as a lush lawn.

A little over 3 years, this is our 4th summer here. The yard wasn't well maintained, there are many dry spots. I'll ask my neighbors on either side of me, they're original homeowners. The three houses were built at the same time. Maybe they'd just happen to know.

As seabornman said, sometimes the leach areas actually appear dryer and sometimes the expected Peg Bracken effect prevails. In either case, the field IS sometimes visable. Back to the OP picture, there doesn't appear to be any area for a leach field there. The field is probably several hundred feet long, snaking back and forth under a cleared area. In my community, some of the leach fields are hundreds of feet away from the house. I have a long lot. My neighbor behind me has a pipe running perhaps 400 feet under the edge of my property, passing by two other homes (and then mine) and finally reaching his leach field adjacent to mine.

Did not find the above post to be helpful at all.