If you've been researching accessory dwelling units, you've probably encountered the terms ADU, DADU, A-ADU, AADU, and others. The terminology can be confusing, but the underlying concept is straightforward: ADU is the umbrella term for any secondary dwelling on a residential lot. DADU is one specific type — the detached version.
Understanding the differences between ADU types matters because each has different costs, timelines, regulations, rental potential, and practical implications for how you use your property. The right choice depends on your lot, your budget, your timeline, and what you want the unit for.
This guide explains all four ADU types available in Bellingham, compares them side by side, and helps you determine which option best fits your goals. If you already know what you want, our free feasibility study will confirm what your specific lot can support.
The 4 Types of ADUs
Detached ADU (DADU)
Also known as: Backyard cottage, granny flat, laneway house
A completely separate, standalone dwelling built on the same lot as the primary home. Has its own entrance, foundation, walls, roof, and utility connections. Maximum privacy and independence for both households.
Advantages
- Maximum tenant/occupant privacy
- Highest rental income potential
- No disruption to main home during construction
- Adds most property value
Considerations
- Highest construction cost
- Longest construction timeline (10-12 months)
- Requires separate utility connections
- Uses backyard space
Attached ADU (A-ADU)
Also known as: Home addition, in-law suite
An addition built onto the existing primary home with its own separate entrance. Shares at least one wall with the main house. Can share some utility connections, reducing infrastructure costs.
Advantages
- Lower cost than detached (shared wall and utilities)
- Faster construction (6-9 months)
- Can share heating systems in some designs
- Preserves backyard space
Considerations
- Less privacy — shared wall means some noise transfer
- Lower rental rates than detached units
- Construction disrupts main house occupants
- Design must match existing home exterior
Basement Conversion
Also known as: Basement ADU, mother-in-law apartment
Conversion of an existing basement into a separate dwelling unit with its own entrance, kitchen, bathroom, and living space. Requires meeting light, ventilation, ceiling height, and egress requirements.
Advantages
- Lowest cost if basement is already finished or partially finished
- Fastest timeline (3-6 months)
- No change to exterior appearance
- No reduction in yard space
Considerations
- Limited by existing basement layout and ceiling height
- May require window wells for egress
- Can have moisture and waterproofing challenges
- Less privacy than detached or attached options
Garage Conversion
Also known as: Garage ADU, converted garage
Conversion of an existing garage (attached or detached) into a dwelling unit. The existing structure provides the shell, and the conversion adds insulation, utilities, interior finishes, and a kitchen/bathroom.
Advantages
- Lower cost — structure already exists
- Faster timeline (3-6 months)
- Detached garages provide good privacy
- Foundation and framing already in place
Considerations
- Lose vehicle storage
- May require significant structural upgrades
- Existing footprint limits layout options
- Often needs new foundation work for residential use
Side-by-Side Comparison
Here is how the four ADU types compare across the factors that matter most to Bellingham homeowners:
| Category | DADU | Attached | Basement | Garage |
|---|---|---|---|---|
| Typical Cost | $200K-$400K+ | $150K-$300K | $80K-$175K | $100K-$175K |
| Construction Time | 10-12 months | 6-9 months | 3-6 months | 3-6 months |
| Privacy Level | Highest | Moderate | Lower | Moderate-High |
| Rental Income | $1,500-$2,500/mo | $1,200-$2,000/mo | $1,000-$1,700/mo | $1,200-$2,000/mo |
| Disruption to Household | Minimal | Significant | Moderate | Minimal |
| Yard Impact | Uses yard space | Minimal | None | None |
| Design Flexibility | Maximum | Moderate | Limited | Limited |
| Complexity | New construction | Addition to existing | Existing space conversion | Existing structure conversion |
| Utility Connections | Separate laterals required | Can share with main home | Shares with main home | May share or need new |
| Property Value Added | $150K-$300K+ | $100K-$200K+ | $75K-$150K | $80K-$175K |
All figures are estimates for Bellingham and Whatcom County. Actual costs, timelines, and rental rates vary by project. See our pricing page for more detail.
When to Choose a Detached ADU (DADU)
A detached ADU is the premium option — highest cost, but also the highest returns and the most design flexibility. Choose a DADU when:
You Want Maximum Rental Income
Separate structures command the highest rents because tenants value having their own entrance, walls, and yard space. In Bellingham, a one-bedroom DADU rents for $200-$500 more per month than an equivalent attached or basement unit.
Privacy Is a Priority
Whether for aging parents, an adult child, or a rental tenant, a detached structure provides the best separation. No shared walls means no noise transfer and complete independence for both households.
Your Lot Has the Space
DADUs require adequate setbacks from property lines (typically 5 feet in Bellingham) and sufficient lot coverage remaining after accounting for the main home. Lots over 5,000 sq ft in Bellingham typically have room for a meaningful DADU.
You Want Complete Design Freedom
Because a DADU is a standalone structure, you have full control over the floor plan, style, orientation, and features — without the constraints of working around an existing building. Visit our design page for inspiration.
When to Choose an Attached ADU
An attached ADU is a practical middle ground — lower cost and faster construction than a DADU, with more privacy and independence than a basement conversion. Choose an attached ADU when:
- Your lot is too small for a detached structure — tighter lots may not have room for a DADU with proper setbacks, but an addition to the existing home works because it extends from the current footprint.
- You want to save on utility costs — sharing a wall and potentially some utility infrastructure (water heater, HVAC) reduces both construction and operating costs.
- You want to preserve your backyard — an attached ADU extends from the side or rear of your home without placing a second structure in the yard.
- The unit is for family — an attached ADU with an interior connecting door (kept locked when renting) gives aging parents or adult children easy access to the main home while maintaining separate living spaces.
Learn more about our attached ADU services.
When to Choose a Conversion
Basement and garage conversions are the most cost-effective and fastest ADU options because the structure already exists. The trade-off is less design flexibility — you are working within the constraints of an existing building.
Basement ADU
Best when your basement has adequate ceiling height (minimum 7 feet finished), a viable location for windows or window wells (egress code), and access for a separate entrance. Older Bellingham homes with full basements are often excellent candidates.
Key requirement: The basement must have or be able to accommodate egress windows, natural light, and a separate exterior entrance. Some basements need structural modifications to meet ceiling height requirements.
Garage Conversion
Best when you have a garage you no longer need for vehicles (or are willing to give up). Detached garages make particularly good conversion candidates because they already provide separation from the main home — giving you near-DADU-level privacy at conversion prices.
Key requirement: The garage structure must be sound enough to convert to residential use. Some garages need foundation upgrades, structural reinforcement, or new roof framing to meet residential building codes.
Bellingham Allows 2 ADUs Per Lot
Under HB 1337, Bellingham must allow at least two ADUs on residential lots. This opens up powerful combinations:
DADU + Basement Conversion
Maximum income potential: a new backyard cottage plus a basement unit can generate $2,500-$4,500/month combined. The basement conversion adds income at relatively low cost since the space already exists.
DADU + Attached ADU
Ideal for multi-generational families: parents in the attached unit (easy main-home access), rental tenant in the detached cottage. Or rent both for maximum income.
Garage Conversion + Attached ADU
If you already have a detached garage, converting it gives you one unit while adding an attached ADU gives you two. Cost-effective combination since one unit is a conversion.
Two DADUs
For larger lots with sufficient coverage allowance: two separate backyard cottages provide maximum rental income and flexibility. This requires a larger lot to accommodate setbacks for both structures.
The key constraint is total lot coverage — all structures (main home + ADUs + garage) cannot exceed the maximum impervious surface percentage for your zone. Our feasibility study calculates exactly what your lot can support. Read more in our two-ADUs-per-lot guide.
Not Sure Which ADU Type Is Right?
Our free feasibility study analyzes your specific lot and tells you which ADU types are possible, which combinations work, and what each will cost. It takes 48 hours and costs nothing.
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Frequently Asked Questions
What does DADU stand for?
DADU stands for Detached Accessory Dwelling Unit. It's a fully independent, code-compliant dwelling built as a separate structure from the primary home — commonly called a backyard cottage, granny flat, or laneway house. The "detached" part distinguishes it from an attached ADU (built as an addition to the main house) or a conversion ADU (built within existing space like a basement or garage).
Can I have both a DADU and an attached ADU on the same lot in Bellingham?
Yes. Under Washington's HB 1337, Bellingham must allow at least two ADUs per residential lot. This can be any combination: two detached ADUs, one detached plus one attached, one detached plus one conversion, or other pairings. The key constraint is total lot coverage — all structures combined cannot exceed the maximum lot coverage percentage for your zone. Our feasibility study calculates exactly what combinations your specific lot can support.
Which ADU type has the best return on investment?
It depends on your situation. Detached ADUs (DADUs) generally have the highest rental income potential ($1,500-$2,500/month in Bellingham) because tenants value the privacy of a separate structure. However, they also have the highest construction cost ($200,000-$400,000+). Garage conversions have the best ROI percentage because the structure already exists — you can create a rental unit for $100,000-$175,000. Attached ADUs fall in between. The best choice depends on your lot, budget, timeline, and goals.
Is a DADU more expensive than an attached ADU?
Generally yes. A detached ADU requires its own foundation, complete structure, separate utility connections, and site work — all of which add cost compared to an attached ADU that shares a wall (and often utilities) with the main house. A typical detached ADU in Bellingham costs $200,000-$400,000+, while an attached ADU of similar size runs $150,000-$300,000. However, detached ADUs command higher rents ($200-$500/month more than attached units) and add more property value because of the privacy and independence they offer.
Find the Right ADU for Your Property
Every lot is different. Our feasibility study identifies which ADU types work on your specific property, what combinations are possible, and provides a preliminary budget for each option.
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