ADU Sitework & Utility Costs: 10 Hidden Costs

Jun 26, 2023 | 0 comments

When planning an ADU, it’s important to know where the ADU will fit and how big it can be… but that is just the beginning. There are a host of factors to consider when determining the best location for your ADU that will provide an optimal outcome & value. Does the ADU fit within required setbacks? How will the ADU be accessed? Are there views, weather patterns, or environmental factors to consider? Is there a more favorable area to build due to slope, vegetation or utilities?

The vertical building cost for your granny flat is easy to know upfront… the structure and foundation are new construction, so we know exactly what materials and labor will be required to build the guest house to plan specifications. However, the sitework costs for two homeowners building exactly the same ADU plan could vary by tens of thousands of dollars… and we want to determine if any of those costs could apply to your project as early as we can. Site work is often the least definable cost in an accessory dwelling unit project. Unlike new primary residences, ADUs are not required to have civil engineering plans, which would give specifics on how and where utilities can tie in.

Based on our experience working on ADUs, we have determined that $25K will cover sitework for a flat lot, provided we can perform tie-ins all within the bounds of your property. Our standard design/build process is to first perform a robust assessment of all utilities on site (prior to even starting floor plans) to determine, size, location and serviceability. With that info, we are able to provide firm pricing for site work so you have that information available very early in your project. Read on to understand the most common additional costs and whether they will likely apply to your property. Below are 10 sneaky costs to watch out for when building your ADU in San Diego.

1. SEWAGE & LONG UTILITY RUNS: What is the type, location and distance of utilities?

If your property is on a septic system, it is almost certain that the ADU will need a new separate system. Design + installation for a new septic system can be handled as part of the overall ADU build process. A septic system for an ADU will typically cost an additional $30-40K. Read more about septic systems on our blog. If your property has sewer access, the tie-in location to the existing sewer will impact your ADU location.

While all other utilities (water, gas, electric) can run uphill, sewage is the only utility which typically operates using gravity. During our feasibility study, we locate the desired location of the ADU, determine the nearest point for tie-in, and calculate the required “fall” for proper sewage flow. If a proper “fall” cannot be achieved, we identify the need for a sewage pump, which costs an additional $5K.

Additionally, long utility runs can add costs. If we have to run utilities beyond what is included in our basic sitework package, you may also see additional costs of up to $200 per linear foot that we have to run — so this could add several thousand depending on how large your property is.

Learn more about water, sewer, and gas connections for your ADU.

2. SLOPE: How much of a slope will need to be flattened for the ADU?

Our example costs for ADU sitework are based on more or less flat lots. If your property includes a steep slope, terracing, or a hillside, the sitework cost of creating a buildable pad for the casita can be more expensive. If you have a very sloped lot, the grading and retaining costs could be an additional $20-30K+, both for the work itself and for a grading plan. This cost will be highly dependent on property specifics.

As a rule of thumb, you can look at the “building corner differential” for your ADU; what is the maximum difference in elevation across the corners of your ADU?

  • More than 1′ and up to 2′ differential: Deepened Footings – $4500 price (this is also triggered when ADU within 10′ of a slope or structure)
  • More than 2′ and up to 4′ differential:
    • Grading (when space permits) – ranging between $10-20K
    • Raised Foundation (when space constrained) – ranging between $10-20K
  • Beyond a 4′ differential: Custom design best suited for site constraints may involve a combination of methods, price varies


3. SOILS EXPORT & GRADING: For grade changes, how much will be required to get to a buildable pad?

Unless you have a very flat lot, you will need to do some grading work to prepare the site for an ADU. Also, if we cannot “lose” the soil on-site, we will need to export soil from the site. So if you have a space-constrained lot, you may need to budget for soils export. The grading & soils export cost will be highly dependent on property specifics, roughly $5-10K. Read more about grading plans and soils reports for ADUs.

4. SURVEY: Is your lot challenging with unclear boundaries?

In many cases, cities are requiring a survey to plot out the location of the ADU and confirm that it will be built within a permitted area. Carlsbad, Oceanside, and San Marcos all require building verification surveys in certain situations. Additionally, if you have a challenging lot with features like easements, hillside, or an irregular shape, a survey may be needed to determine lot lines and the appropriate setbacks. A building survey typically costs $4-10K. Read more about when you may need to get a survey for your ADU.

5. SOILS REPORT: Do you need a soils report?

Chula Vista, Poway, Encinitas and Vista always require a soils report, while Carlsbad and San Marcos require soils reports for units of 500 sqft or more. Declining a soils report means that no soils compaction testing will be performed by a Geotechnical Engineer, and the ADU will be built to the most stringent UBC design standards & thresholds. A soils report typically costs $3K and can be obtained as part of the ADU build process with SnapADU. Overexcavation required by the city due to soils conditions can add another $4-5K. Read more about when you may need a soils report for your ADU.

6. SUFFICIENT ACCESS: How much space is available for bringing in construction equipment?

We will need to access the ADU location with construction equipment and materials. If there is not sufficient access, we will likely need to do some of the digging by hand and/or crane materials onto the site. If there is less than eight feet of clear vertical access (e.g. eaves could restrict the clearance) to the back yard or if the surrounding soil cannot support heavy equipment, this will typically increase cost by about $5-8K.

7. STRUCTURES & SURROUNDINGS: What accessory buildings and landscaping are in or near the desired building area?

You may have existing sheds, large trees, retaining walls, or hardscape in the area where you want to build the ADU. We will need to plan for relocation or removal accordingly, the cost of which will be highly dependent on the site specifics. For instance, removal of a large tree can be $2-4K.

8. SEPARATE ELECTRIC PANEL: Do you have a separate meter for the ADU?

When you add an ADU, you will need to add a separate electric meter for the ADU at a cost of $10,500 (this is something that was not historically enforced, but SDGE now requires ALL new construction ADUs to have a separate meter). There are additional factors that may require more work on your electrical service and you can read more about electric panels and service upgrades for ADUs.

9. SOLAR: Is there solar power on the primary dwelling?

Most additions and new buildings in California are now required to meet net-zero electricity guidelines. Particularly for larger units and/or in hotter climate zones, this means your ADU plan must include a solar energy system with capacity to generate enough electricity to offset the ADU’s electrical usage. You can meet this requirement by adding more capacity to an existing system or adding a new system for the ADU. A solar panel system for an ADU typically costs between $12-18K installed, though each city has their own requirements and your ADU size will also impact the size of system needed. Also note that you can take advantage of a 30% federal tax credit on the cost of the solar system in the year it goes into service, so you will get back thousands at tax time. Read more about solar on ADUs.

10. SPRINKLERS: Are there sprinklers in the main house?

ADU regulations require that, if the main home has sprinklers, any ADU built on the property must also have a fire sprinkler system. This is usually the case in high fire zones. If you go >1200 sqft – which you can in Poway – you will also need fire sprinklers. A sprinkler system design & installation in an ADU costs $5-10K depending on the size of the dwelling unit. Another factor here is that adding a sprinkler system will increase the water line size requirements, which can add significant expense. Read more about fire ratings & sprinkler systems.

Other watch-outs that can result in additional costs

  • Easements. If you have an easement on your property, you will need to request a title report to verify the location of the easement. Read more about easements.
  • Coastal zone. Properties in the coastal zone will usually require minor use permits. Read more about the coastal zone.
  • High fire overlay zone. These areas may require additional construction requirements such as boxed in eaves and fire-rated windows. Read more about fire requirements.
  • HOAs. In areas where an HOA is in place, they may require some upgrades in the exterior of the ADU, such as a tile roof or custom trim work. Read more about HOA requirements.
  • In-person submission. Certain jurisdictions require in-person submission of plans, which means more printing and labor costs as we must produce paper plans and physically take them into the city. Check your city specifics on our service area page.
  • Storm water management. Some cities, like Vista and Encinitas, require site drainage and storm water mitigation for ADUs. Finish grading work may be required to create a bio-swale, drainage basin (bioretention basin) or other contouring work to handle on-site storm water, which will cost around $5-10K in most cases. Read more about stormwater management for ADUs.
  • Illegal construction. If your property has existing structures that are unpermitted, you may need to bring them into compliance with current building codes.
  • Existing floor plan measurements. Poway requires a floor plan of the existing home, while some HOAs and Coastal processes may as well. This costs around $750 to produce.
  • Metering. Some cities, like Oceanside, require a private water meter, which will run about $500.
  • Swimming pool. If you have a swimming pool on your property, you will need to secure it in order to get occupancy for your ADU.
  • Construction loan coordination. If you are obtaining a construction loan, there will be significantly more required paperwork and inspections. SnapADU handles the processing of loan documents during design and coordinating with the lender inspections and draws during construction, which adds about $3000 in cost.

So what’s next?

Whew. It’s a lot to consider, and we haven’t even covered how the design features of a custom ADU will affect the build cost. The type of siding, roofing, windows, floor plan layout, and finish materials will of course impact your price as well. Our standard designs are designed to keep these build costs predictable. If you do choose to go semi-custom, we will ensure you have a clear picture of cost every step of the way.

We help clients take all of the quantitative and qualitative factors into account on their ADU project. Our objective during initial discussions is to determine a desired size and footprint of the ADU. Then we produce a preliminary site plan showing the layout of the structure on the site. When clients move forward with a feasibility study, their preliminary site plan will be reviewed in depth by our team for regulatory, architectural, structural, building cost and sitework considerations that could suggest a better layout for the ADU. We review and finalize this design & layout – as well as the final guaranteed price – with our clients before moving on to produce construction drawings that will be required for submission to the city for permits.

We’d love to help you weigh all the critical factors that will impact the cost to build your ADU. Take 60 seconds to fill out a Property Assessment to get started. It’s never too early to get us involved to talk through your options.

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The insights shared in this technical blog are derived from our extensive expertise in the field, encompassing hands-on experience in design, development, and implementation. We are committed to delivering accurate and current information; however, it’s important to acknowledge the dynamic nature of the ADU space, which may lead to changes affecting the details discussed in this blog.

SnapADU is dedicated to empowering our readers with valuable insights and practical knowledge, enabling them to navigate the constantly evolving accessory dwelling unit landscape. We invite you to connect with our experts to receive personalized guidance and to remain informed about the latest advancements within the field.

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