Site Preparation

Every site has to be prepared to have an ADU get placed on it. There are a few key steps to get ready to prepare your site for an ADU.

  • Clear debris + Items
  • Learn where your electrical and plumbing lines are located and make them accessible
  • Upgrade your electrical panel to accommodate increased capacity for your ADU
  • Level your working site so the surface is flat
  • Remove any trees/shrubs as needed (note: you may need a permit to remove a tree in your city)


Foundations are critical as they bear and distribute the load of your home. They also prevent groundwater and soil vapors from getting into your home to protect it from mold & warp. Below are a few of your most popular choices:

  • Basement

    • Most expensive option.
    • Provide a lot of value, add square footage to your home without increasing your footprint.
    • If the water table runs higher on your property – probably not the best location.
  • Slab

    • Most common type of foundation in CA
    • Usually one of the cheapest options
    • Less maintenance than other foundations
    • Earthquakes + expansive soils (Clay) will cause these foundations to crack. Post tension cables in the slabs have helped prevent cracks.
    • Ground should be faily level, not a great choice when there is a slope
  • Crawlspace

    • Can be used in place of or with a slab foundation
    • Footings are poured then cinder blocks are placed on top to support the house’s walls.
    • Positioned a few feet above the ground (hence a ‘crawlspace’, which leaves an opening underneath the structure.
    • Plumbing repairs are easier because you have a space beneath the home.
    • Provide protection against moisture
  • Pier and Beam

    • Piers are drilled into the soil to rest on rock which in turn support beams to support the house.
    • House is less likely to shift around as a result of movement in the soil directly underneath your home.
  • Helical Piles

    • Provide foundational support for structures of all types and sizes. Just like old-fashioned pile-foundations, helical piles (also referred to as screw piles, helical piers, helical anchors, etc.) are embedded deep underground in order to disperse the enormous weight which sits atop them.
    • Multiple piles are installed to create a platform, on which a building can rest.

    The foundation you use will depend on your soil type, geographic conditions (like if you’re in a flood zone/earthquake prone area), and the type of home you build (additional reinforcements are needed for a two-story vs a one-story). Make sure you work with a licensed contractor with experience working in your jurisdiction – they’ll know which foundation types will work for your property.

Rough Framing

Rough framing is the skeleton of your home. Most homes are built out of a wood frame, but other materials chosen include steel or concrete.

Most ADUs (and residential homes) are framed with wood because it is:

  • less expensive
  • easy to work with
  • doesn’t require welding
  • weighs less
  • contractor’s and planning departments are familiar with it


Framing Structure

Each framing structure essentially consists of two components: studs and joists. Studs are the vertical components which provide stability for your wall coverings. Horizontals elements, joists, run the length of the floor or between walls. Joists support your ceiling and floors.

Walls can be load-bearing and non-load bearing. A load bearing wall supports a load from above (like another floor or roof). A non-load bearing wall, or a partition wall, doesn’t support weight.

Exterior wall sheathing helps reinforce your walls by keeping the studs together. Typically this is done with drywall (gypsum board), or plywood.

Rough-In Work

You’ll have two main components to in rough-in, electrical and plumbing.

Electrical Rough-In

Electrical rough-in means your cables have been pulled through the studs and framing and are inserted into wall and ceiling boxes. Light switches, lights and devices are not attached yet.

Plumbing Rough-In

All water supply and drain pipes are run through open holes in your framing and all pipe connections are made.

No sinks or fixtures have been installed yet at this phase.

If the house has a concrete slab foundation, plumbing rough-in happens before concrete is poured.

Waste pipes are completed first because they are larger than the water supply lines. All lines are pressure-tested before deemed complete.


Insulation, at its most basic property, is any material used to fill the spaces (above ceilings, gaps, behind walls) in your home to reduce heat flow by reflection/absorption.

When the temperature is hotter outside your house, heat moves in. When the temperature in your house is warmer – heat moves out.Insulation helps slow the move of heat so it isn’t as warm in the summer or as cold in the winter. Proper insulation will increase your home’s energy efficiency and reduce monthly heating/cooling bills. provides a detailed breakdown of the different types of insulation available.

Types of Insulation (provided by


Your roof is your upper layer of your home and is made up of more than just shingles. Here are the parts of a roof:

  • Roof Plane: The surface of your roof. Flat but at an angle.
  • Ridge: Top or the peak of the roof, where two roof planes meet.
  • Valley: Where two pitched roof faces connect and project inward. Always at a lower slope than adjoining roof planes.
  • Gable: Triangular wall underneath where two roof planes meet.
  • Eave: Where the roof will hang over the edge of the exterior wall.

What are the roofs themselves made up of? Here are the componenets:

  • Shingles: thin piece of material attached on your roof to help prevent water from entering at the edges.
  • Flashing: Thin sheet, usually made of metal, installed around any vertical surface intersecting with the roof plance (like the surface of a chimney). These are used to help prevent water from entering the edges of any vertical component.
  • Underlayment: A fabric-like barrier applied directly to the decking to protect the shingles from any resin the decking may release and to serve as a water barrier in case water gets underneath the shingles.
  • Ice and Water Protector: Another thin layer of material installed beneath shingles to help protect from win-driven rain and against ice dams.
  • Roof Frame: Contractors will add a wooden frame for the roof on top of the house, this roof truss will serve as the skeleton.
  • Roof’s Edge: A roof will end with an eave or a rake. An eave is a horizontal edge where the roof hangs over the exterior wall while a rake is a sloped edge where the roof hangs over the exterior wall.
  • Solar: has a detailed list of how solar works, why you should go solar and the cost of solar. It’s important to know that every ADU in California is now required to have solar due to new Title 24 requirements.


Siding is the protective material attached to the exterior side of a wall or house. Any structure that is internally framed must have siding. It helps to protect your home from inclement weather while keeping your home properly insulated. Here are some of the types of siding you can choose from:

Vinyl Siding


  • Vinyl: popular choice because it’s well known, less expensive than other materials and comes in a wide range of colors. Not as durable as other options.
  • Wood Siding: Expensive, but gives an aesthetic appeal, however you can easily paint your home to change it’s look when desired.
  • Manufactured Wood Siding: Looks like real wood siding, at a fraction of the cost. The only downside is to make sure you keep moisture out of the siding to prevent mold and mildew.
  • Metal Siding: Low maintenance with resistance to fire, however not the best siding option if living in a wet or coastal climate due to it’s potential for rust.
  • Fiber Cement Siding: Very durable, but is more expensive than other siding options with not as high of an R-Value.
In the next chapter, we’ll get into what the costs are to actually build an ADU. We’ll even include a budget breakdown of an actual project in California. Chapter 6: Costs.