ADU Basement Conversions

An accessory dwelling unit (ADU) is an extra living space suitable for multigenerational living of your family members, passive income, and many other purposes. There are several different types of accessory dwelling units — detached, attached, junior ADUs, garage conversions, etc. A less-known but also rather high-demand ADU type is a basement conversion. It’s one of the simplest, quickest, and cheapest ways to create an ADU.

In our article, we’ll have a detailed look at this unusual accessory dwelling unit. We’ll talk about the key aspects of the conversion process — the necessity of building permits and housing regulations for such ADUs, their legality in California, the main steps of their construction, their biggest advantages, and so on.

Are Basement Conversions Legal

Is converting your basement into an ADU legal in California? In many of its regions, yes. However, most houses in this state don’t have basements — but if they do, homeowners often make a living space there. Sometimes, they do not get any permits or hire specialists for this task. As a result, such ADUs often lack safety and structural integrity, so we don’t recommend it.

Since there aren’t many basement ADU conversions in California, information about them is also quite scarce. Nevertheless, we surely know that rules and regulations for them also exist. As with any other ADU, a basement accessory dwelling unit should have its own separate entrance, a kitchen, a bathroom, insulation, plumbing, and electric systems.

A widespread problem of basement ADUs is that they need to have egress windows located at least 44 inches high. There are clerestory windows in many basements, but they don’t fit this requirement. Egress windows should not only be a source of natural light but also serve as a potential fire escape.

You can solve this problem by making a window well. It’s a metal, wooden, or plastic piece that resembles the letter U. To install it, you’ll need to dig a hole around the window and put the well there so that the place in front of the window will remain clear.

Is a Permit Necessary for Basement ADUs

Many people wonder whether a building permit is necessary for a basement conversion project. Yes, such structural modifications are also needed. An experienced designer usually does everything possible to get the permit approved in the local jurisdiction. If everything’s OK, obtaining it takes from 3 to 4 months. Also, don’t forget that the permitting process can involve impact fees, so include them in your ADU budget.

Converting a Basement into an ADU

To create a basement ADU on your property, you’ll need to divide your goals into two categories: short-term and long-term. First of all, you’ll have to decide what you’re going to use your ADU for — extra income, hosting guests, a living space for your family member, a mini-office for freelance work, or anything else. An ADU’s purpose should play a big role in its planning.

After that, contact a design specialist with significant experience in ADU projects. It’s super important. With a skillful ADU designer, you’ll be able to obtain the permit much easier.

In addition to that, you’ll need to find a suitable ADU general contractor. Since they’re extremely popular nowadays, don’t postpone looking for a GC. When you find one, sign up in the schedule as soon as you can. Some contractors offer a free consultation.

Basement ADUs. Main Advantages

Since this type of ADU project doesn’t require any additional construction, it’s way more cost-effective than other in-law suites. However, in addition to the lower overall costs, these ADUs have many other benefits.

Let’s look at the most important advantages of basement dwelling units:

• A basement is an already existing space. So, a basement remodel doesn’t need any excavation and building. It can save you much money and time;
• You won’t need to make electricity and plumbing systems because the water and electrical hookups are already there. And even if they aren’t — they’re much easier to conduct than in a separately standing backyard cottage;
• Probably, you already have a living space there — you’ll only need to bring it into compliance with all the building codes and get a permit;
• Lower energy cost. The temperatures tend to be more moderate below the ground level;
• Typically, people who choose basement ADUs live in close-in neighborhoods without many infill property lots. Such neighborhoods are very popular among tenants;
• You won’t have to diminish your valuable yard space, while an attached or a detached structure requires additional square footage in your backyard;
• Discreet density without altering the look of your neighborhood — other people may not even notice your additional dwelling unit from the outside;
• You can reintegrate the ADU space with your main residence if necessary.

Main Tips for Basement ADU Conversions

In order to successfully convert your basement into an ADU, you’ll need to follow several recommendations.

Here are some of them:

1. Decide how high your ceiling will be. Usually, the local regulations cover this topic. When calculating the ceiling height, take into account things like joists, beams, proper insulation, and so on. If the height turns out to be too low, it’s possible to dig your basement out, but it will cost you an extra $10,000-$20,000. And in case you do it yourself, it will be extremely tiresome;
2. When making the floor plan, try to create a spacious feel to make the interior of your ADU seem larger. It’s especially important for small spaces;
3. Check whether there’s radon in the basement. It’s a harmful radioactive gas, which, in high concentrations, can lead to serious illnesses. Also, look for termites, mice, rats, and other parasites. If you’ve found them, contact a pest control service. Another thing that you should search for is rotten and moldy surfaces. Take them out of the basement if it’s possible, or at least try to remove the mold;
4. Make a private entrance leading from the main house to the ADU. The average cost of installing it is approximately $10,000;
5. All the main utilities of your house (such as an electrical panel, a furnace, and a water heater) are probably already in your basement. You’ll need to make them easily accessible from the primary residence. Move them closer to the interior stairs or make a hallway connecting those stairs with the utilities. The cost of moving a heater ranges from $1,000 to $2,000, a furnace – $2,000-$5,000, and an electro panel – $3,000-$10,000;
6. Often, humidity levels in basements are high, so before converting your one into an ADU, you’ll need to dry it up. Otherwise, there can be serious problems such as damp air, stain-covered walls, or mold growth. During rainy weather, the moisture will only get worse. Water can seep into the basement, causing even more issues. To avoid it, you’ll need drainage solutions — for example, downspouts, gutters, or a sump pump. For homes situated on slants, a great option is French drains;
7. Think about how you will share the house space with others. It’s especially important if you’re going to generate additional income with your ADU. In this case, you’ll have to share it with complete strangers, not relatives or friends. Regardless of the sound insulation thickness, you’ll be aware that underneath your house, there’s an independent living space for other people. Also, the walkway will be in common use, and you’ll have to maintain the house together. So, if you value your personal space greatly, a basement ADU for rent may not be suitable for you.

So, a basement ADU is a very convenient, inexpensive, and easy-to-construct accessory dwelling unit that has numerous benefits. Sadly, many people in California don’t have the possibility to construct it due to the lack of basements in their homes.

Frequently Asked Questions — FAQ

Do I need planning permission to convert my basement?

Yes. You’ll need all the necessary building permits for converting your basement space into an accessory dwelling unit. However, some homeowners convert their basement into an ADU without going through a permitting process, following building codes and local regulations. But such a basement remodel can lack structural integrity, be unsafe and require various unexpected expenses.

How much is a basement ADU conversion?

Like the price of any ADU, a basement conversion’s price depends on its square footage and other factors. However, since a basement ADU doesn’t require additional construction costs, it’s much cheaper than an attached or a detached unit. Typically, its price begins at $120,000. Basically, it’s in the same price category as garage conversions.

How long does a basement ADU conversion take?

Converting your basement space into an ADU is much faster than an ordinary ADU construction, since it won’t require any additional building job on your property. In some cases, it can take up from 2 to 3 weeks. However, just as the price, it may depend on the basement’s square footage and many other things.

How to convert a basement into a living space?

Converting your basement into a living area requires some important steps. First of all, you’ll need to measure it (for this, you can use a tape measure), make a floor plan with a bathroom and a kitchen, install proper insulation and egress windows, protect the basement from moisture, check it for radon, pests, mold, and rot. Make all the basement utilities easily accessible from your primary residence. For the maximum safety and convenience of your new ADU, hire an experienced designer and a general contractor.