Five Things I’d Do Differently if I Built My ADU Today

My accessory dwelling unit (ADU) was completed almost three years ago, and I’ve been living in it ever since. An ADU is a secondary housing unit on a single-family residential lot. Also known as a granny flat or guesthouse, an ADU can be a new detach separate house, a converted garage, an apartment above a garage, an attached unit, a carve out from the main house or a converted basement. What I chose to build was a 700 square foot detached separate house.

With 20/20 hindsight, there are a few things I would do differently if I were building it today. If you’re thinking about or planning to build an ADU, I hope my lessons learned help you in your own designing and building process.

1) I’d pick a different property.

The terrain and slope of your property play a critical role in the cost of building an ADU. The property and house I bought are on a hill and the lot slopes downhill from north to south and from east to west. A sloped lot requires retaining walls, additional hardscaping and can add to foundation and sewer costs. Other factors, such as ease of access, can factor into the construction cost. If you’re looking for a house and property to add an ADU, property selection is an important factor. See my post Select the Right Property for Your ADU and learn from my mistakes.

2) I’d be my own contractor for the second half of the project.

I wouldn’t suggest this for everyone, but if you have some building/remodeling experience and know good subcontractors, you can save yourself a fortune. And you’ll have better quality control over the aesthetic finishings on both the interior and exterior of your ADU.

Leave the building of the foundation and structure to the experts unless you are a licensed contractor. Be sure your architect and contractor are experienced in designing and building ADUs. It’s a whole different animal. Hire them to design and build the structure including the foundation, framing, roof, plumbing within the walls, electrical and heating/cooling systems.

Building a house is like making a cake, you don’t want to make any mistakes in the baking process. That’s the critical portion. The rest is frosting. If you’re good at project managing, you can hire your own plumber, electrician, drywaller, floor installer, painter and carpenter.

I really wish I had gone this route as I was not happy with the quality of the painting and the installation of the tiling, electrical outlets, paneling, shelves and hardware. Even with a contractor handling the finishing touches, I spent many more hours supervising than should have been necessary. Especially when I was paying the contractor 25 percent to manage the project. I had to be on site daily to check the work, spot mistakes and be sure they were corrected. They installed three-inch baseboards instead of six-inch. They started to lay the flooring in the wrong direction, which would have made the rooms look smaller. Not one shelf, light switch or piece of hardware was straight when first installed. The paint on the kitchen cabinets started chipping after one year.

If you have the time, being your own contractor for the second half of the build will not only save you a ton of money, but you’ll have better quality control over all the finishing touches on your new ADU.

3) I’d hire an interior designer or contractor who could provide more guidance.

There are dozens and dozens of decisions to make when you are building a house and most of them must be made months before the foundation is even laid. From kitchen faucets to fireplace inserts, bathtubs to wall heaters, hardware to light fixtures. I have a lot of imagination. I know what I like aesthetically. And I’m good at visualizing things. But I’m not building houses every day of the week. There are a lot of considerations when selecting products to be sure they are the right fit and function for your home. Almost every decision took hours of online research, in-person browsing and/or asking experts so I could make an informed decision.

The contractor and architect had some recommendations, but several of their suggestions turned out to be duds. Like the under-the-floor heating in the bathroom, which they promised would heat the entire bathroom. It didn’t, because they didn’t insulate the floor. I hired an electrician to disconnect the under-floor heating and installed a wall heater, which makes the room toasty warm in no time.

The contractor didn’t install an on/off switch for the energy recovery ventilator (ERV) system, telling me it wasn’t necessary. The ERV brings in fresh air, which is required in a new, energy-efficient home because the structure is so tightly sealed. But it also brings in all my neighbor’s barbeque smoke. Thankfully, I had an on/off switch installed (another expense) right before the smoke from the horrendous wildfires in 2020 made Portland air unbreathable.

If I could have afforded it, I would have hired an interior designer to advise me on many of the decisions, such as where to place the kitchen faucets so the soap dispenser doesn’t empty onto the wood countertops. It would have been worth it to avoid the hours of internet research and decision making. I now have a wealth of interior design knowledge but nowhere to use it unless I build or remodel another house. We shall see!

4) I might have purchased a prefab house and saved myself a fortune.

Although I love all the unique features of my home — the large windows, the Victorian fireplace mantel, the 14-foot ceilings — it wasn’t cheap. Various types of prefab, modular or kit houses have been around for over 100 years, but at the time I built my ADU there weren’t many companies creating prefab homes of 800 square feet or less. Today you can find a variety of smaller, prefab houses varying in cost and quality, with some only a step up from a mobile home, while others offer elegant designs with high-end features.

A prefab house saves money because there are no architect, engineering or design costs. You simply select a preexisting design that works for your property. But that’s the challenge. With smaller lot sizes, city setbacks, an existing house and a sloped lot, prefab houses rarely fit the bill.

5) I’d have built in more storage or built a storage shed.

Living in a small house, I’ve learned one can never have enough storage. As much as I have purged, there are still things I’m holding on to. It would have been nice to have an insulated storage shed attached to the house so things I stored there would survive a cold, damp Oregon winter. A shed also makes a nice workshop where one can tinker on projects and not have to put everything away at the end of the day. So far, I haven’t needed to add a shed. I have a storage attic and I also store items in the basement of the main house.

All in all, I’ve been very happy the past three years living in my ADU. It’s been a great place to cocoon during the pandemic. I hope this post is helpful for your own ADU planning. If you have any questions, post them below.