Attached ADUs make sense on Sunnyvale lots where the back yard is too small to give up, where the parcel is narrow, or where a single story overlay limits how much you can build vertically and every square foot has to be efficient. Building off the existing house borrows a wall, a foundation edge, and utility runs that already exist.
That is a meaningful cost difference, and on tight lots it is often the difference between a project that works and one that does not.
Attached refers to the construction, not the way the unit lives.
If your home is one of Sunnyvale's Eichlers, an attached unit is a specialist job. Post and beam structure does not accept new loads the way conventional stick framing does, and the connection detail matters structurally rather than cosmetically. The low slope roof has its own flashing and drainage requirements where old meets new.
Then there is the slab. Many of these homes heat through radiant tubing cast into the concrete. Running new plumbing for an attached unit can mean cutting that slab, and doing it without mapping the tubing first is how a heating system gets destroyed. We treat that as a survey step, not a guess.
Code requires fire rated separation between the ADU and the main house. Meeting that rating does not deliver quiet, and quiet is what makes living wall to wall with a tenant workable over years rather than months.
We insulate the shared cavity properly, decouple framing where the layout allows it, and route plumbing so the unit's noise does not travel into your bedrooms. How we handle these assemblies is covered on our ADU construction page.
Sharing utilities is the financial argument for attached units, and capacity is the catch. A Sunnyvale house from the fifties or sixties was not wired for a second kitchen and a second full set of appliance circuits. A service upgrade is common, and we check and price it at the start rather than mid project.
Structurally, an attached ADU is an addition. New foundation edge, framing tied into existing framing, a roof junction that has to stay watertight, and an envelope that performs. The difference is that the interior has to function as an independent residence.
We build additions with the same crews and standards, which is why this is natural work for us. Our home addition services page covers the overlap, and our explainer on what an ADU house is helps if you are still deciding.
Whether attached is the right answer depends on your wall, your panel, your zoning, and the house you already own. Contact our team and we will walk your Sunnyvale property.
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