Thinking About an ADU? 8 Questions to Answer Before You Build
A practical guide to feasibility, cost, design, permitting, septic systems, construction, and what happens after you decide to move forward.
An accessory dwelling unit can create a closer place for a parent to live, give an adult child more independence, add rental income, or make a property work better for the future. But once the idea starts to feel real, the questions arrive quickly.
Can an ADU actually fit on the property? What will the full project cost—not just the home itself? How disruptive will construction be? And what should you pay for first?
During our recent ADU FAQs & Common Barriers webinar, Small Home Advisors Jeff Bowman and Tim Swenson answered the questions they hear most often from homeowners across Maine, Massachusetts, and New Hampshire. Here are the biggest takeaways.
1. Can I build an ADU on my property?
Possibly—and more properties can support ADUs than many homeowners assume. The real question is not simply whether ADUs are allowed in your state. It is what type of ADU works on your particular property.
An ADU might be detached, attached to the main home, or created by converting existing space. The best option depends on local rules and the physical conditions of the lot.
A first review should look at:
Zoning requirements, including setbacks and lot coverage
Available space and the relationship between existing structures
Water, sewer or septic, and electrical connections
Slope, drainage, tree clearing, and driveway needs
Access for construction equipment and, for a modular project, a crane
State laws may establish a baseline right to build certain types of ADUs, but local requirements and property conditions still shape what is practical. That is why a property-specific conversation is more useful than a simple yes-or-no answer.
2. What will the ADU really cost?
There is no single price that applies to every ADU. Size and finish selections matter, but they are only part of the total project cost.
In many projects, the home itself represents half—or sometimes less than half—of the complete budget. The rest can include site work, the foundation, utility connections, permitting, design and engineering, landscaping, driveway work, and other property-specific needs.
This is why it is important to compare all-in project estimates, not just the advertised cost of a home or floor plan. A lower home price does not necessarily mean a lower completed project.
Before focusing on a particular design, get clear on the purpose of the ADU. Is it for a family member, long-term rental income, a mix of both, or simply more flexible living space? Knowing who the home is for helps determine which features are worth prioritizing and which costs may not be necessary.
3. Will construction take over my yard?
Construction will affect the property. There will be excavation, equipment, delivery access, and an active work area around the future home. Good planning can limit surprises, but no builder should pretend the ADU simply appears overnight.
The advantage of modular construction is that the home and the site can be prepared at the same time. While the foundation and utility work are happening on the property, much of the home is being built indoors in a factory. A typical Backyard ADUs home arrives roughly 80% to 85% complete.
On-site construction often lasts about two and a half to three months, although site conditions, permitting, weather, and municipal requirements can make that window shorter or longer. On set day, the modules are placed on the prepared foundation by crane and can typically be roofed and made weather-tight very quickly. Finish work continues after the set.
The most helpful step is to identify anything on the property that must be protected—a favorite tree, garden, stone wall, driveway area, or view—and discuss the construction path early.
4. How do I avoid getting overwhelmed by design decisions?
An ADU may be small, but it still comes with plenty of choices. The goal is not to make every decision at once. It is to make them in the right order.
Start with the parts that have the greatest effect on feasibility and budget: the site, utilities, home size, layout, and major accessibility needs. Finish selections should come later, once the project has a realistic framework.
Design and pricing also need to stay connected. Ten seemingly small changes can add up to a meaningful budget increase. A structured process provides “bumpers,” as Tim described during the webinar, so the design stays aligned with the homeowner’s goals.
Think about daily life rather than trends. One person may need a highly accessible bathroom. Another may value a larger kitchen, more storage, a front porch, or a window positioned toward a favorite view. The right ADU is designed around the person who will live there.
5. Who handles permitting and local rules?
Permitting feels confusing because it often is. Rules can differ by state, town, zoning district, and even by whether the ADU is attached or detached. Requirements are also changing as more communities update their housing regulations.
Your town’s planning, zoning, or code-enforcement staff can be a helpful starting point. When you work with Backyard ADUs, our team reviews the applicable rules, communicates with local officials, prepares permit applications, and represents the project through the process when needed.
Attached and detached ADUs are not always treated exactly the same. Fire separation, setbacks, sprinkler requirements, and the way a new foundation connects to an existing home can all affect the design. In some cases, a detached ADU is actually simpler and less expensive because it does not have to be physically integrated with the existing structure.
6. What if my septic system is a barrier?
Septic is one of the most common concerns, especially in rural areas and on the Cape. It can affect where the ADU sits, how many bedrooms are possible, how much of the yard is disturbed, and the total budget.
The key questions include the age and capacity of the current system, its approved bedroom count, and its location relative to the proposed ADU. Depending on the property, options may include:
Connecting the ADU to an existing system, if capacity and local rules allow
Installing a separate septic system for the ADU
Replacing the existing system with a new shared system
Using an approved alternative treatment system
In some municipalities, formally converting an existing bedroom to another use to keep the permitted bedroom count unchanged
Composting toilets may be allowed in some places, but they do not eliminate the need to address gray water from sinks, showers, laundry, and kitchens. A septic professional or engineer can help identify the right option, but their work should be coordinated with the larger site plan.
7. What should I pay for first?
It is easy to spend money in the wrong order. A homeowner may order a septic design before choosing a build location, only to discover that the proposed septic system occupies the best place for the ADU. Or they may pay for a survey that is not detailed enough—or is too old—to satisfy the town’s permitting requirements.
Before commissioning major site work, confirm three things:
The property appears capable of supporting the project
The likely total budget is within reach
There is a realistic financing path
You do not need every answer before taking the first step. You do need enough information to reduce the risk of paying for work that will have to be repeated or discarded.
8. What happens after I decide to move forward?
At Backyard ADUs, the process is organized into three main phases: feasibility, design, and construction.
Feasibility
The feasibility study examines zoning, setbacks, utilities, site access, grade, clearing, drainage, driveway needs, and construction logistics. It is designed to identify the property’s constraints early and show how those conditions are likely to affect the project and budget.
Design and planning
Once the project appears feasible, the team develops the site and home design, coordinates surveys and engineering as needed, works through permitting, and helps the homeowner make selections while keeping pricing visible.
Construction
The home is built off-site while the property is prepared. After the modules arrive and are set on the foundation, the team completes the on-site connections and finish work.
A typical project takes about a year from the beginning of feasibility to move-in, but timelines vary. Conservation review, septic design, surveys, municipal approvals, financing, weather, and personal deadlines can all affect the schedule.
A few quick answers from the webinar
What does “modular” mean?
Backyard ADUs uses volumetric modular construction. The home is built in three-dimensional sections off-site, rather than assembled from flat wall panels. Cabinets, doors, lighting, wiring, plumbing, and many finishes are already installed before the home reaches the property.
Are the homes energy efficient?
Backyard ADUs homes are typically all-electric and built with high levels of insulation, air sealing, heat-pump systems, and energy-recovery ventilation. Actual energy costs vary by household habits, utility rates, location, and the final design.
Does Backyard ADUs handle permitting?
Yes. In most projects, the team manages the permitting process so the homeowner and local officials have a consistent point of contact.
The best first step is a conversation
You do not need to arrive with a finished floor plan, a new survey, or every financing detail resolved. Start with the property, the reason you want an ADU, and a realistic sense of budget and timing. From there, the right early questions can show you what is possible—and help you avoid expensive work in the wrong order.
If you are considering an ADU in Maine, Massachusetts, or New Hampshire, the Backyard ADUs team can review your goals and help you understand the next step.
