ADU Sitework & Utilities: 10 Sneaky Costs to Watch Out For

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: What is the type and location 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 will locate the existing sewer, determine the nearest point for tie-in, calculate the required “fall” (downhill slope of sewage pipe required to achieve proper flow) to the desired location of the ADU. It may be the case that nowhere on your site can achieve proper “fall.” In such cases, we identify the need for a sewage pump to be included in the build out, which typically costs an additional $5K.

Additionally, 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.

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.

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.

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

In most cases, ADUs do not require a survey. They have been waived, but 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 $4-6K. Read more about when you may need to get a survey for your ADU.

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

Most jurisdictions do not require ADUs to have a soils report, though Chula Vista and Carlsbad do require one. 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 $5K and can be obtained as part of the ADU build process with SnapADU. 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. SIZE OF ELECTRIC PANEL: What amperage is the existing electrical panel?

When you add an ADU, you will need to connect to your existing electric panel. If the electric panel is not large enough to accommodate the additional requirements of the ADU, you will need an additional electric panel, which costs $7500, plus $2500-7500 if a new wire pull and conduit upgrade is needed.

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

All additions and new buildings in California are now required to meet net-zero electricity guidelines, which 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 about $9K installed (eight panels for 1000 sqft or smaller), though each city has their own requirements.

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 in an ADU costs $3-8K depending on the size of the dwelling unit. Another factor here is that adding a sprinkler system will increase the water line size requirements, mentioned earlier in this list.

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
  • 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.
  • 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.
  • Illegal construction. If your property has existing structures that are unpermitted, you may need to bring them into compliance with current building codes.

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. 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 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. Use the calendar below to schedule a free consultation. It’s never too early to get us involved.