By Ben Bogie, Director of Building Science, BPC Homes
It usually comes up at the kitchen table. Mom is on her own in a house that’s too big for one person, the driveway is a mess every time it snows, and she’s 40 minutes away when something goes sideways. Or it’s your daughter, her husband, and a toddler, who could easily buy a place across town but would rather be close enough for Sunday dinner and the occasional 6 a.m. babysitting emergency. Then somebody says it out loud: what if we put a little house out back?
And everybody gets quiet, because they’re picturing two years of trucks in the driveway, a contractor who stops returning calls, and a budget that doubles somewhere around month four.
I get it. A mother-in-law suite (or in-law apartment, or backyard cottage, or ADU if you want the zoning-office term) is a small house, but it’s still a whole house. It needs a kitchen, a bathroom, heat, power, fresh air, water, and somewhere for everything that goes down the drain to end up. That’s exactly why who builds it matters so much.
I’ll be upfront about something. We build these at BPC, but my buddy Josh Salinger, who runs Birdsmouth Design-Build out in Portland (Oregon, not Maine… one Portland on this coast is plenty), has built a whole lot more of them than I have. So when we got talking recently, I picked his brain and we compared notes. Josh has a great talk on what makes ADUs different from every other kind of residential building, and most of what follows borrows from it, with a Connecticut accent.
Who’s moving in?
Before anybody draws a floor plan, the first question is who’s going to live there, and for how long. A cottage for Mom at 78 and a cottage for your son and his new wife are two different buildings.
If the plan is aging in place, design for the version of Mom who’s 88, not the one who still walks three miles a day: one level, a no-step entry, wide doorways, a curbless shower with blocking for grab bars, and switches and outlets she doesn’t have to bend for. None of that costs much when it’s drawn in from the start, and all of it costs a lot as a retrofit. Most people start looking for aging in place builders after something happens. It’s a lot easier to do it before.
Universal design just means a home that works for whoever lives in it. If the kids move in first, the priorities shift toward a little more room and some real privacy (on both sides of the yard, trust me). A good design can do both, so the cottage that works for your daughter’s family now can work for Mom later. That’s what makes it a true multigenerational home.
Out back, over the garage, or downstairs
A detached ADU, the backyard cottage most people picture, gives everyone the most privacy and the most design freedom. It’s usually the most expensive option too, because you’re building a whole little house from the dirt up and trenching utilities back to it.
An in-law suite addition shares a wall with the main house. You save a wall of siding and windows, but now you need a fire-rated separation between the two homes, some thought about sound, and careful surgery where new meets old. (Old houses are rarely as square as the drawings say.)
A garage conversion ADU looks like the bargain, since the walls and roof are already there. The catch is what’s under them. Garage slabs often have no vapor barrier, the footings may be minimal or missing, and 2×4 walls built to keep the rain off a car don’t come close to what a bedroom needs, so by the time all that’s fixed the bargain has mostly evaporated. Building over a garage has its own list: fire and sound separation, car fumes, stairs, and serious air sealing in the floor.
And then there’s the basement. This is Connecticut, so we have a lot of basements, and plenty of them are damp. A basement apartment can work beautifully, but water management comes first, followed by egress windows (which usually means cutting concrete), ceiling height, natural light, and noise between floors. Skip the water part and you’ve built a very nice mold farm.
So, what does it cost?
This is the part everyone wants first, so let’s not be coy about it. How much does an ADU cost? More than most people expect, and size has less to do with it than you’d think.
Josh makes a point I’ve seen play out too: a 600-square-foot ADU and an 800-square-foot ADU cost about the same, because the expensive stuff is there either way. Every one needs a kitchen, a full bathroom, heating and cooling, ventilation, electrical service, and plumbing. Pack all of that into a small footprint and the price per square foot runs higher than a full-size house. Think camper van versus house. The van is smaller, but it still has every system the house has, crammed into less space.
Price per square foot is a slippery number anyway, so ask what the budget actually covers. A real one accounts for:
- Design and engineering
- Permits and town fees
- Utility hookups, plus any upgrades to the main house’s electrical panel, water service, or septic
- Site work like driveway repair, drainage, tree protection, and erosion control
- Demolition of an old garage or shed, including testing for asbestos or lead in older buildings
- Finishes, landscaping, and final cleanup
Septic deserves its own mention. A lot of our clients from Newtown up into Litchfield County are on septic, and adding a bedroom can mean the system has to be checked against the health code or upgraded. It’s not a dealbreaker, but it’s a line item you want on the table at the first meeting rather than discovered in month four.
That’s how we run it at BPC. We put the whole-project budget on the table early, scope and all, and we talk openly about building in a buffer for the unknowns, usually around 20% of time and budget. Surprises can still happen, but they happen on paper, before anyone’s swinging a hammer.
Get the builder in the room early
Here’s where a lot of ADU projects go sideways. Someone draws a lovely cottage, it goes out to bid, and the numbers come back way over. Josh calls it a game of telephone: when design and budget get worked out in separate rooms, nobody finds out the plan doesn’t fit the money until it’s expensive to change.
Our custom home building process works the same way on a cottage as on a full-size house. We bring in an architect or designer we trust (or yours, if you already have someone), and the builder sits at the table from the first sketch. In what we call our Align stage, we nail down your design goals, performance targets, and budget before anything is locked in. On a small building that usually means simplifying, with a cleaner roofline, a smarter footprint, and fewer jogs in the walls. Simple is cheaper to build, easier to make airtight, and easier to maintain.
This is also where your town comes in, with plenty to say about setbacks, size, height, and parking. Height limits are the sneaky one, because they can push you toward vaulted ceilings, which changes how you insulate the roof. A good team sorts all of that out up front, so you’re not spending your Saturdays at Town Hall.
Four people in a Subaru
Okay, this is the part where I get a little nerdy, so bear with me.
Put four people in a Subaru on a January morning and watch what happens to the windows. That’s a small space with a lot of people breathing in it, and the moisture has nowhere to go. An ADU is a lot like that Subaru. Showers, cooking, laundry, and people all happen in a small volume, and a well-insulated little building has a tiny heating load, which means less drying power than a big drafty house. Josh calls it a high people-to-volume ratio, and it’s one of the biggest technical differences between an ADU and a regular house.
Get it wrong and you get condensation, stale air, and mold. Get it right and the place feels fresh and comfortable all year, which is the whole point of a healthy home. Right looks like the same stuff that makes high performance homes work: a continuous air barrier so humid indoor air can’t sneak into the walls, a good water-resistive barrier with a vented rainscreen behind the siding so any water that gets in can get back out, and a balanced fresh-air system (an ERV) that brings in clean air and pushes stale, wet air out all day long.
Then there’s heating and cooling. Small, tight buildings need small equipment, and an oversized system short-cycles, never pulls the summer humidity out, and wears out early. We run real load calculations and size the system to the building, which on a well-built cottage is often one small heat pump, planned from day one instead of squeezed into a closet at the end.
Built to be boring
Here’s a question I’d ask every family thinking about this: who’s going to take care of it? If Mom’s cottage needs new trim in ten years and a furnace tune-up every fall, that’s your job now, and the whole point was to make life easier.
So we build these to be boring. That means durable siding over a rainscreen, and flashing details done right, because the carpenter installing the flashing matters more than any product on the shelf. It means going all-electric, so there’s no oil delivery to schedule, no gas line to run, and no combustion appliance next to where Mom sleeps. And it means a heat pump water heater instead of something that needs fussing over.
I’m not going to pretend it’s zero maintenance. The ERV has filters that need changing a few times a year, and that’s about a five-minute job. Compared to the alternative, though, it’s pretty close.
We also put it in writing. Our home builder warranty, the BPC Green Warranty, covers comfort (within 3°F of 72°F, year-round), sets a ceiling on energy use, and spells out ventilation performance. That’s the kind of peace of mind that matters when the person living there is someone you love.
Living next to the job site
Building in someone’s backyard means living next to the job site for months. Josh had a whole slide on this, and I laughed through most of it because it’s true everywhere: the chatty homeowner (we love you, but the crew needs to work), the “can you just…” request that quietly turns into a day, the dog that gets out, the neighbor worried about parking, and the delivery truck that can’t fit down the driveway.
A good crew plans for all of it with a real site plan for staging, parking, and the dumpster. They protect your trees and gardens, set up and clean up every single day, give the neighbors a heads-up before the loud stuff starts, and make sure you have one person to call who actually picks up.
When it’s done, we test the building, commission every system, and walk you (and Mom) through how it all works, so she knows what the thermostat does before she unpacks the first box.
Back to the kitchen table
What you’re after is Sunday dinner without the drive, someone close enough to check on Mom after a storm, and grandkids who can walk across the yard to see their grandparents. A well-built mother-in-law suite gets you there. Build it right and it becomes a forever home for whoever lives in it next, whether that’s Mom, the kids, or you someday.
It’s a small house and a big decision, and it shouldn’t come with a big headache. If you’re in Fairfield, Litchfield, or New Haven County, or just over the line in Westchester, Putnam, or Dutchess, and this idea keeps coming up at your table, give us a call. We’ll walk the property with you and tell you what we’d do.
And a big thanks to Josh Salinger and the crew at Birdsmouth Design-Build for letting me borrow his notes. If you’re out in Portland (the Oregon one) and thinking about an ADU, go see them.