This Old House S47 E22: Walpole ADU Framing, Shared Utilities, and a Plumber's Flags

The ADU behind Dan and Jill's 1971 Walpole ranch is framed now, which means Season 47's last project has entered the part of construction that photographs badly. Episode 22 is about connective tissue: a wall knocked out between the old ranch and the new addition, a 2x4 interior wall tied into a 2x6 exterior wall, shared drain lines under a shared floor, and a brand-new underground electrical service. Kevin O'Connor also flies to Atlanta to look at how another city handles accessory dwelling units, and that segment delivers the most concrete ADU cost and code talk I've heard on this show.

Walpole Ranch with ADU: What Changed Since Last Time

The old garage is gone. Its slab and footings never made it below the frost line, and when the crew priced out underpinning against simply pouring a new foundation on the same footprint, the new foundation won. That decision was the cliffhanger of the previous Walpole post on the frost problem and the mechanical room, and Episode 22 opens with the answer already poured and cured.

The new addition behind the garage pad runs roughly 65 feet long and 25 feet wide, and it's really three projects stacked end to end. Closest to the original house, the primary bedroom is getting bigger, with an en suite bath and more closet. In the middle sits a bonus room of about 500 square feet, shared by both households, with a door on each side so nobody has to walk through somebody else's kitchen. At the back, just under 900 square feet of ADU: one bedroom, two full baths, laundry, living room, kitchen, and its own access to the yard.

Kevin O'Connor and general contractor Kevin take down the wall between the old primary bedroom and the new space, and here's the detail that tells you this demo was planned rather than improvised. Nobody lays protection on the floors, because the hardwood is coming out of the entire house anyway. Plastic sheeting and ram board are the giveaway that a crew is cutting into something they intend to keep.

The This Old House helper scorecard last checked in at 5 for 6 back in the Carolina comeback episode, and Kevin holds his own here: he sets up, he swings a hammer, and he doesn't ask a question that makes Tom Silva put his tool down. Call it a push.

Tom Silva's 2x4 Meets 2x6 Problem

Tom and Jared walk through a framing transition that looks like a detail and behaves like a rule. The wall grabbing the stair opening coming up from the basement is an exterior wall, so it's 2x6. It needs the depth for insulation and to receive sheathing. The interior wall running away from that corner is 2x4, because there's no exterior insulation to accommodate.

The reason the two sizes have to line up is cabinetry. A wall is going away further down the run, and the finished face that remains has to be one flat plane for new cabinets to mount against. Mix thicknesses carelessly at that junction and you spend an afternoon shimming cabinet boxes and a lifetime looking at a crooked reveal.

Carlos marks the top and bottom plates from the outside face of the wall, not the inside, because the sheathing needs to break on layout at 16 inches on center. Marking from the wrong side is a classic way to end up with a sheathing seam landing mid-bay, which means extra blocking and a call to the engineer's details page.

The header over the stairway door gets its own solution. Rather than hunting down a thicker member, Tom builds it from three 2x8s with two pieces of sheathing filler sandwiched in. Three 2x8s measure 4 1/2 inches; two half-inch fillers bring that to 5 1/2, which matches the 2x6 wall exactly. He also points out jack studs and king studs at each side of the opening, the two kings with a jack carrying the header. Studs cut at 85 1/8 for the top and bottom plates, everything assembled on the deck and tipped into place. It's the kind of sequence that takes four minutes on camera and most of a morning on a real site.

Richard Trethewey, a Plumber Named Bob, and the Flag Trick

Richard opens the utilities question he set up in Episode 21. For this ADU, in this town, at this moment, water and sewer can be shared with the main house. Richard makes clear that these rules shift by state and municipality and will keep shifting, so anyone planning an ADU should treat shared utilities as a local question, not a general one.

The water line branches off to feed the ADU. The existing cast iron sewer runs out to the street, and the new ADU drain ties in with PVC. Down in the basement, plumber Bob is laying out two ADU bathrooms and a kitchen, which is where the episode gets genuinely instructive.

Bob works alone, with no apprentice. So before he drills a single hole through a floor system, he drills pilot holes and drops landscape flags through them, labeled lav, toilet, laundry waste. Then he walks downstairs and reads the flags against the structure. If a flag lands under an I-beam, he knows before the drill does.

One of them did land badly. The laundry waste pilot came down through the top of an engineered joist, and Bob measured from below, moved the layout upstairs, and sent the pilot back down before committing. Richard's commentary is the teachable part: drilling through the top of engineered lumber removes the material doing the structural work. Manufactured joists carry their load in the flanges, and every one of them ships with a hole chart that tells you where you're allowed to cut. The episode doesn't walk through the chart, so I'm adding that context, but the demonstration is why you check first.

Bob also uses a laser level to set the centers of the toilet drains, which lets him cut and fit a 3-inch 90 and its lateral with the layout already proven. He labels every branch on the main. Richard, delighted, calls the laser the best unpaid apprentice on the job, and admits an old dog learned something.

Worth noticing: the two households share water and sewer, but not electricity. That's a deliberate split, and it comes up next.

Underground Service: 200 Amps In, 400 Amps Out

The old service was overhead, strung from a pole out front, across the driveway, to the corner of a garage that no longer exists. So the service had to be rethought entirely. Electrician Sean Brennan found a pole behind the house, which lets the feed run underground to the back corner of the ADU.

The upgrade goes from 200 amps to 400, delivered as two 200-amp services with two meters. That gives the ADU and the main house separate utility bills, which is the arrangement most people building a rental or a family unit eventually want. It also requires a work request filed with the utility naming the pole and the service size, which Sean describes with the dry practicality of a man who has done this before.

Trench depth is the code item here. The National Electrical Code calls for 18 inches, and Sean asks his site crew for 24 so the 90-degree sweeps at the pole and at the house sit entirely below grade rather than poking up into the lawn. Conduits go in on a level bed of sand, because pipe sitting on a void eventually cracks. Where the run needs to turn, the crew uses a PVC hot box to heat the pipe until it's floppy, then bends it and cools it with water so it holds its shape and stays round. Kink the pipe and you've created a spot where the wire won't pull.

Backfill happens in stages, and this is the part homeowners never see: six inches of sand over the conduits, then direct-burial warning tape, then a call to the local wire inspector. He verifies the sand depth and the trench before anyone is allowed to bury the run. That staged inspection is normal for underground work, and it's why a service change takes longer than the trenching calendar suggests.

Atlanta ADUs Since 2017: Four Properties, One Reality Check

Atlanta passed an ADU ordinance in 2017. More than 350 have been permitted since. Kevin tours four projects with architects Meg and Christina: a garage conversion still in design, a cement-block building mid-construction, a finished new-construction unit, and a one-year-old ADU in Decatur.

The design-phase property is the one to study. It's a 1930s bungalow with an old one-car garage and outbuilding on the lot, and because the structure already exists, the project is grandfathered into setbacks that sit right on the lot line to the side and about three feet from the fence at the back. Build new in that same yard and you're 15 feet off the rear and 7 feet off the side. That gap between existing and new is the single most valuable thing in this segment.

Inside the finished 550-square-foot unit, the small-space moves are honest: vaulted ceilings that add volume without adding square footage, an all-in-one washer and dryer, a smaller refrigerator, a hot plate in place of a range, and a loft over the bedroom. The loft stairs are steep. Christina explains that ADUs have their own stair code, which is how that rise passes. I'd flag that as jurisdiction-specific rather than universal, and exactly the kind of thing to confirm with a local building official before drawing anything.

Cost and schedule land where these conversations always should. Expect about 12 months start to finish, with roughly six months of design, layout, and permitting ahead of six months of building. Budget $300 to $350 per square foot, which puts a 750-square-foot Atlanta ADU somewhere around $230,000 to $260,000 depending on finishes. Decatur allows up to 1,000 square feet, 24 feet tall, two stories, and 10-foot setbacks, and that project used 525 square feet plus a sleeping loft. It runs as a pool house in summer and a rental the rest of the year.

DIY Confidence Scale

2 out of 10. Watching is free and genuinely educational. Doing is not.

A supervised interior wall demolition is the only item here a confident homeowner might attempt, and only after someone qualified confirms the wall isn't load-bearing and the utilities are dead. Everything past that is trade work: drain, waste, and vent layout around engineered joists, a 400-amp service upgrade with a utility work request and a staged inspection, trenching to code depth, and a 2x4-to-2x6 framing transition that has to stay flush for cabinetry.

Hire the plumber, the electrician, and the carpenter. The flags are a great idea to steal for your own renovation, though. Drop one through a pilot hole before you drill anything.

Additional Resources

• Watch on PBS
• Official This Old House episode page

The Walpole job doesn't pause. The preview shows Zack installing the most popular siding in the country, Jared explaining that the homeowners want a product they aren't repainting every four or five years, a factory tour of a building assembled from a pile of wood into something road-ready, and Richard tucking HVAC between ceiling joists. The garage framing and the rest of the back addition are still waiting.