New Septic System Installation in Woodside, CA
Full tank and field systems, engineered from scratch
A new septic system installation is different from a replacement job. It usually applies to new construction, a property that never had a septic system, an addition or ADU that pushes an existing system past its designed capacity, or a full teardown-and-rebuild replacement where both the tank and the field are being engineered together from the ground up. Because it is not working around an existing layout, a new installation gives more flexibility in siting the system for the best possible soil and slope conditions on the property.
When You Need a Full New System
- New construction. Homes being built on lots without existing city sewer connections need a septic system designed and permitted before occupancy.
- ADUs and additions. Adding bedrooms or a separate dwelling unit often exceeds what the existing system was designed to handle, requiring an upgraded or entirely new system.
- Total system failure. When both the tank and the field have reached the end of their life, or the existing field location can no longer meet current setback and soil requirements, a full new system is designed and installed together rather than patched piece by piece.
- Major property renovation. Large-scale landscaping or hardscape changes sometimes require relocating a septic system entirely.
How Sizing Works
System size is based on the number of bedrooms (used as a proxy for expected occupancy and water use), the results of a percolation test that measures how well the specific soil on the property absorbs water, and the available usable area on the lot. Hillside and estate properties in Woodside, Portola Valley, and Los Altos Hills often need larger or more engineered systems than flat-lot suburban homes of the same size, because slope and soil conditions are less forgiving.
The Design and Approval Process
- Percolation and soil testing. Determines how quickly water absorbs into the specific soil on your lot, which drives the entire system design.
- System design. A licensed designer lays out tank size, field type (gravity or pressure distribution), and placement based on the soil results, slope, setbacks from wells and property lines, and county requirements.
- San Mateo County Environmental Health permit. The design is submitted for county review and approval before any excavation begins. This step is often the longest part of the timeline, typically two to six weeks depending on the county's queue.
- Excavation and installation. Tank and field are installed to the approved design specifications.
- Final inspection. The county inspects the completed system before it can be used, and before occupancy is approved for new construction.
Planning for the Future
Because a new installation is designed from scratch, this is the point where it is worth thinking beyond current needs. A system sized only for today's household can force an expensive redesign later if an ADU or addition is added down the road. Discussing future plans with the contractor before finalizing the design is one of the more overlooked ways to save money over the life of the property.
Common Questions About New System Installation
How long does it take to install a brand new septic system?
The physical installation itself is often one to two weeks. The county permit review beforehand typically takes two to six weeks depending on the queue, so total project timelines usually run one to two months from initial evaluation to final inspection.
Do I need a percolation test even if my neighbor already has a septic system?
Yes. Soil conditions can vary significantly even between adjacent lots, especially on hillside terrain, and San Mateo County requires site-specific percolation testing for new system design regardless of what neighboring properties have.
Can I add a bedroom or ADU without upgrading my septic system?
It depends on how much capacity your current system has relative to the addition. In many cases, additional bedrooms or an ADU require either an upgraded system or confirmation from the county that the existing system has adequate reserve capacity.
What happens if the percolation test comes back poor?
Poor-draining soil does not necessarily rule out a system, but it usually means a pressure distribution or engineered system is required instead of a conventional gravity field, which increases cost but is still a permittable solution on most lots.
See the full cost guide for new installation pricing, or if you are dealing with an existing failing system rather than new construction, see leach field repair and tank replacement.