View Full Version : Connecting a 2nd building to an existing septic system

July 1st, 2009, 09:33 PM
I own a 1 acre property with an existing 3 bedroom home on it. I'd like to build a separate structure (two car garage with a small 700 square foot, 1 bedroom apartment above it) and am wondering what my septic system options are:

1. Is it possible to just hook into the existing septic system? The tank would be about 120 feet away from the structure, so I'm guessing I'd need a pump. Is it even feasible?

2. If I have to install a separate tank for the detached structure, can I hook it up to the existing drainfield? The drainfield sits roughly between the two buildings.

3. Since it's a one bedroom apartment, are there cheaper options for dealing with septic waste than building a full two bedroom septic system (e.g., are there smaller, or self-contained above ground solutions that might work)?

Thanks in advance for any guidance you can provide!

July 1st, 2009, 11:37 PM
This is usually controlled by your local health department. Septic systems are sized by the number possible users. It is likely the your current system will too small the add another dwelling unit to it.

July 2nd, 2009, 12:41 AM
Thanks Joe. That's a great point. I'll check with my health department.

On an acre of land, I suppose I have enough room to install a second septic system, but I've never heard of a one-bedroom septic system, and am wondering is there's a less costly, less invasive solution available to me.

Is anyone aware of any simpler solutions for smaller (one bedroom, one bath, small kitchen, apartment sized washer/dryer) structures?

July 2nd, 2009, 12:56 AM
How old is your existing drainfield? If it is close to 40 years old, then you're nearing replacement time. That would be a good time to redesign things to accomodate both buildings.

In general, I think it is cheaper to have one septic system shared by two houses than two separate systems, even if a common drainfield.

If you need to redo the system, I'd put a large tank (perhaps 500 to 1000 gallons larger than what you have now) midway between the two buildings and run an appropriate sized drain field from that. It would be ideal if you could gravity feed this tank from both buildings, as that will eliminate pumps. You'll then need a larger drainfield than what you had before.

If you want to use the existing system with minimal mods, no one can answer your question without more data. Need to know how many bedrooms is the existing house? How large is the existing tank? How much drainfield do you currently have (total length)? What is your soil perk rate? What does your health department require now for your house (tank and drainfield size)? What will they require if you add another 1 bedroom building to it?

Most builders don't put in extra margin in a septic design, and health departments tend to get stricter over time requiring your same house to need a larger system. As joed said, your current system is most likely too small even for what you have connected to it now. Adding another building would most likely be impossible without adding tank and drainfield capacity.

July 2nd, 2009, 02:24 AM
Mark, thanks for the terrific advice. Hey! I notice you're in my neck of the woods (my house is in Redmond). It's an interesting situation, and I'm trying to figure out how to optimize my plans. Here's a bit more background...

The house was built in 1978. It's a three bedroom home with what I'm assuming is a 3 bedroom septic system. The soil perks great (flat lot on top of a hill, with zero drainage issues). It's an acre lot about 350 feet long east/west and 130 feet wide north/south. Access to the lot is from the west side.

The current tank is located towards the north west end of the lot about 50 feet in from the west edge, and I'd like to put the garage/apartment towards the southwest corner (about 25 feet in from the west edge). Putting a new tank in between the two structures would be ideal. The only issue I have is that in a couple of years, I'd like to tear down the old house and pour a foundation for a custom home at the center of the lot. Planning ahead, I have two concerns:

1. The current leach field is at the center of the lot where the foundation for the main house will eventually go. At the very least, that means re-locating the leach field when I'm ready to build the main house. In practice that's unavoidable, but it means I can't live in the apartment while building the house until the drainfield is re-located.

2. Putting a larger tank in the middle of the lot to service both structures (or the garage for now, and the main house later) makes total sense. The only practical concern is when the heavy equipment moves in to pour the foundation or deliver materials for constructing the main house, there's a risk they'll damage the tank and/or pipes since they have to cross that area to get to the center of the lot.

I've been racking my brain to think of a cost effective, self-contained solution that works for now, but that gives me expansion flexibility down the road without having to rebuild everything twice.

The more I think about, I'm wondering if a separate (small) tank to service the apartment independently would be better since:

a) I can locate it next to the garage structure (no risk of future construction damage from heavy equipment)
b) the health department probably won't let me connect to the existing 3 bedroom tank anyway
c) I wouldn't have to run a pump 100 feet to the existing tank, and then relocate it later.
d) I may still be able to connect to the existing drainfield (even if it needs to be expanded to accommodate the added load).

I appreciate your insight. As an aside, would you happen to have any idea how much a typical small 2 bedroom septic system costs to install in King County these days? How about a 4 bedroom septic? Is a 1 bedroom septic design even possible and would it save me much over a 2 bedroom septic? Thanks again!

July 2nd, 2009, 02:36 AM
By the way, hopefully my description is clear enough. I can post a picture showing the lot layout if that would help. Thanks for your guidance!

July 2nd, 2009, 07:40 AM
This might be a more cost effective system for you:
You will have to check with your local health department to make sure these are allowed in your area.


July 2nd, 2009, 08:49 AM
Your existing tank might be just fine. You may only need to add more drain filed for the new apartment.

July 2nd, 2009, 10:21 AM
It is critical that vehicles not drive over your tank or drainfield, ever. So if you put in a new one make sure all builders know where it is and you rope it off. Also plan for how the pump truck will get access so it can be within 50' (and preferably less) distance of the tank to pump it out, and not drive over any part of the septic system. Pump trucks are heavy and will crush buried pipes.

It is better to have a separate septic system for each building, but more costly. There is a minimum size the health department will want, so a 1 BR apartment may not have a smaller system than a 2 BR house. Probably a 1000 gallon tank and 200' of classic drainfield or 100' to 150' of infiltrator. I really like the infiltrator stuff, but not sure how much the county health department will allow to replace classic drainfield (Infiltrator says 100' of their system replaces 200' of a classic pipe in gravel field). Mine was a "repair" and not a new install, so don't know how they feel about it. Infiltrator is kind of expensive for just being half circles of plastic with perforations, but it is way less work and you won't have a bunch of gravel trucks driving around.

I think a drainfield will cost about $7K, and maybe $3K more for the tank and piping. Permits take time too. That is for a simple gravity system. If they make you go with a metered system with a pump, or add sand to your field, then the costs can go up quite a bit. If your land drains to a pond, lake, or wetland, then you may not be able to have a system at all. A larger system doesn't cost that much more (maybe 25%). Most of the cost is getting the excavator to show up and dig. A larger tank costs more (figure about $1/gallon), and drainfield is priced by the foot. But a 4 bedroom system may be 1.5 times the size of a 1/2 bedroom system. A large chunk of the costs is non-recurring: dealing with health department and inspections, getting workers/excavators there to do something.

July 2nd, 2009, 11:00 PM
There are a lot of ifs on this subject that only your local board of health can rule on. Example. While the original house was built as a 3 bedroom home, is this true now. Has one of the bedrooms changed to an office or den ? This would eliminate one bedroom load on your septic. Look at what you are really using now in that existing home concerning load. I have a 5 bedroom home. Had one child but planned for more. Got a divorce instead. Now have two bachelors living in a home using 2 bedrooms. Once in a while we find a girlfriend that will put up with us for a while but most often they get mad and move back out. I must be hard to live with I guess. My point is that living conditions change which also changes the loading of that septic system. The local septic inspector can make a ruling on what your current load is and whether you will need a second tank or a second finger system.

Many times your local inspector if septic field in good operating condition and septic tank on good condition will allow you just to add more feet of fingers by adding on another finger to your existing system.

All the above depends on existing conditions and inspector rulings. I would call the board of health and show them your living conditions and wanted future plans to get a final ruling as to what you will need to do.

Good Luck


July 5th, 2009, 01:23 PM
Guys, sorry for the delayed response - I've been busy with a garden shed project over the 4th of July long weekend.

Mark, Joe, everyone else, thank you so much for your insight! You've given me a lot of great information. My next stop is the County Health Department. Hopefully they'll just allow me to hook up to the existing tank. We're only using two bedrooms in the main house, so the load can't be that great. If I need to build a separate system, I'll look at the infiltrator system.

When it comes time to build the larger house, I'm likely going to have to re-design the septic system anyway...I'll re-build and re-locate it to service both structures at that point.

All the best!