---
title: What Goes Into Building an ADU | ADU Construction Services in Portland
description: What does building an ADU actually take? Read on to find out what's included, the steps involved & how long ADU construction runs.
image: https://blog.cooperdesignbuilders.com/hubfs/Blog%20Images/What%20does%20building%20an%20ADU%20include.jpg
---
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# What Does Building an ADU Involve? A Start-to-Finish Overview
Posted by [Josh Glover](https://blog.cooperdesignbuilders.com/author/josh-glover) on Oct 13, 2026, 1:32:14 AM
- **Building an ADU involves planning, design, permits, construction, and inspections.**
- **In Portland, ADUs need a kitchen, bathroom, and proper permits.**
- **The process starts with a site check, then moves into design, and building.**
- **Most custom ADUs take about 6 to 12 months to complete.**
- **The best ADU design depends on how the space will be used.**
What does it take to add a second home to a lot that already has one? That question sits behind most searches for ADU construction services in [Portland](https://cooperdesignbuilders.com/services/adu-builders-portland-or/). As is the case with most bigger projects, the work starts long before the foundation goes in, and it only wraps up after a final city inspection.
Below, we'll take a closer look at what the project usually looks like and how long it runs. Read on!
## What does building an ADU include?
An ADU is a full, livable unit, so building one takes all the same steps as building a new house, just compressed onto a smaller footprint and an existing lot. A full-service project typically covers:
- Site evaluation: Lot dimensions, setbacks, existing structures, trees, slope, and access for crews and equipment
- Design: Layout, exterior character, interior finishes, and how the unit relates to the main house
- Engineering: Structural plans, foundation design, and energy code compliance for the permit set
- Permitting: Building and trade permits, plan review, and the associated fees
- Utility connections: Water, sewer, stormwater, and electrical service
- Construction: Foundation, framing, roofing, mechanical systems, insulation, drywall, and finish work
- Inspections and closeout: City sign-offs at each stage and a final walkthrough
### Does every backyard unit counT as an ADU?
In Portland, a unit counts as an ADU only when it has its own kitchen and bathroom. A home office or studio without a kitchen goes through a different permit process, so it's important to decide early how you'll use the space and what utilities it needs. If you plan for guests to sleep there, the unit needs full permits either way.
### Where do ADU projects run into trouble?
Utilities and permits are where most of the common ADU planning mistakes show up. There's a lot to keep track of, too. A unit at the back of a deep lot often needs a long trench to reach the street, and an older electrical panel may not be able to carry the extra load.
Still, Portland has simplified ADU permitting over the past few years, and an experienced contractor plans for these issues from the first site visit.
## What are the steps to building an ADU?
Even when the finished units look nothing alike, the process stays pretty consistent from one project to the next.
### 1. Feasibility and site evaluation
Professionals visit the site to confirm where a unit can go, what the zoning allows, how utilities would reach it, and whether anything on the lot, like a large tree or a steep grade, will shape the design.
It's also when you'll want to talk through how the unit will be used, since a rental, a home for a parent, a guest house for visiting family, and a home office each push the plan in a different direction.
### 2. Design and engineering
Once the site checks out, it's time to start shaping the design. Early concepts settle the footprint, the layout, and the exterior, and those then get refined into a full drawing set with structural engineering, energy compliance, and finish selections.
In a design-build process, the same team that designs the ADU also prices and builds it. So whenever you pick a material or move a wall, someone's already checking what it'll cost and how it'll actually get built. That keeps the plan realistic as it grows, and you won't end up redrawing half of it once the numbers come in.
### 3. Permitting
Main home or accessory one, a dwelling unit is a dwelling unit in Portland, so an ADU needs a full building permit, plus trade permits for the plumbing, electrical, and mechanical work.
The city also offers [pre-approved plans for detached units](https://www.portland.gov/ppd/residential-permitting/home-projects/accessory-dwelling-units), but custom designs go through standard plan review against the zoning and building codes.
### 4. Construction and inspections
Like any new home, an ADU goes up in a pretty set order. First comes site prep and the foundation, then framing and the roof, then rough-ins for plumbing, electrical, and mechanical, and finally insulation, drywall, and all the finishes. Inspectors check in at set points along the way, and once the final inspection clears, the unit is ready to move into.
## How long does it take to build an ADU?
A custom ADU typically takes six to twelve months to build, though a lot depends on its size, its complexity, and how smoothly permitting goes.
A few things tend to stretch that range:
- Corrections requested during plan review
- Long-lead items like windows, custom cabinetry, or specialty tile
- Site conditions that only show up during excavation
- Utility work that needs coordination with the city
Weather matters, but probably less than you'd think. Rain can slow things down during the foundation and framing stages, but once the shell is closed in, most of what's left happens indoors.
## What design decisions go into an ADU?
There's a lot to decide when designing an ADU, but the layout, the exterior, and the interior finishes are where it really takes shape. It helps to tackle them roughly in that order, since each one makes the next a little easier.
### Layout and placement
The right layout depends on who's going to use the unit. A single open room works great for a home office or one occupant, while a separate bedroom gives a tenant or a parent the privacy they need for long-term living.
A sleeping loft adds space without adding footprint, though the stairs make it a tough fit for anyone with limited mobility. Where the unit sits on the lot matters just as much, since its position decides which way the entry and windows face and how much privacy each building gets.
### Exterior
Portland requires detached units taller than 15 feet to match the main house in finish materials, roof pitch, trim, eaves, and window orientation. Shorter units don't have to follow that rule, but using the same siding and roofing as the house still helps the two buildings look like they belong together.
### Interior Finishes
Flooring, cabinetry, tile, lighting, and color are where [a small ADU gets most of its personality](https://blog.cooperdesignbuilders.com/how-make-small-adu-feel-bigger), and one strong choice, like a patterned tile floor or a statement light fixture, can carry the whole unit.
The right finishes also depend on how the unit will be used and how much wear it has to handle. A rental with changing tenants needs surfaces that can stand up to heavy daily use, while a unit for a parent may call for slip-resistant flooring and clear contrast between surfaces. A guest suite or home office has more room to play with texture and character, since it sees a lot less traffic.
## Who provides ADU construction services in Portland?
COOPER Design Build handles ADUs with the same design-build process behind more than three decades of award-winning kitchens, primary suites, additions, and whole-home renovations for homeowners from [Healy Heights](https://en.wikipedia.org/wiki/Healy_Heights,_Portland,_Oregon) to the far corners of the metro area.
One in-house team carries the project, so the people who design the unit and the crew who build it work from the same plan. That full-service structure covers site evaluation, engineering, permitting, construction, and the warranty that follows, and nobody hands the work off to another firm along the way.
Contact us to talk through what an ADU on your property would involve.
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