How to Build a Multigenerational ADU: ICF Construction From the Ground Up

What you’re about to see is a real multigenerational compound being built from scratch! Three generations, two structures, one property, and a family that did everything right before the first block was ever laid.

And I mean everything. They talked about money. They talked about privacy. They talked about all the nitty gritty and who pays for what when something needs replacing.

Jim’s building the main house which is for the daughter, her husband, and their two kids and her parents are building a 900 square foot ADU in the backyard, detached, private, completely theirs.

This family is genuinely fun and they are so excited for their new compound together!

The planning and all those conversations they had before anyone broke ground, is a big part of why this is going to work so well.

So. Here we go…..

Welcome to the P & E Multigenerational Compound Build Series.

The Multigenerational Compound Build Series: What We’re Documenting

Over the next eight weeks, I’m taking you through every stage of this build.

The ADU first, from slab to move-in. Then the main house. Then how the two structures work together as a compound, not just as two houses that happen to share a lot.

Here’s the full lineup:

• The ADU – slab, ICF walls, roof, interior (that’s this post and the next few)
• The main house – framing, systems, layout decisions
• Compound design – how the two structures relate to each other on the lot, privacy, sightlines, shared utilities
• The conversations this family had before they started – and why those conversations are the reason this will actually work
• Strategy, finanances and alternatives when considering multigenerational living

If you’re thinking about a multigenerational build of your own (whether that’s an ADU, an in-law suite, or a full compound situation) I want this series to be the resource for any of you considering this lifestyle.

Speaking of which, if you’re in the planning stage right now, the Multigenerational Compound Planning Checklist w/ an ADU is the place to start.

It’s free, it’s 96 questions, and it covers everything from budget conversations to privacy design to what happens when someone needs more care down the road.

Grab the free Multigenerational Compound Planning Checklist here.

First, the Old Concrete Barn Had to Come Down

That structure in the photo above had been sitting on this property for decades. Concrete block walls, a corrugated metal roof, and in a spot right where the ADU needed to go.

So before anything could go up, that had to come down.

What you see in the demolition photo is the in-between, the old barn still partially standing in the back while the excavator works through the rubble in the foreground.

It’s messy and a little chaotic but it’s my favorite kind of photo because it captures the moment right before something new starts and makes for the best before and after pictures!

And then there’s the school bus. You’ll notice it lurking in a few of these early photos, an old yellow school bus that had clearly been parked on this property for about 70 years (according to the previous owner) and was patiently waiting for someone to deal with it.

I remember thinking when I first saw it, “did I ride that bus?” This is the town I grew up in so it’s very possible! 😉

The ADU Slab: Why 900 Square Feet Is Bigger Than You Think

The foundation is a slab, 30 feet by 30 feet. Nine hundred square feet.

This ADU was designed by an architect who actually sat down with this couple, learned how they live, how they cook, how much storage they need, whether they want a dedicated guest space for when the grandkids sleep over.

The floor plan was built around their life (not a generic 900 square foot template).

We’ll get into the interior in a later post and I think you’re going to be genuinely surprised by what fits.

One detail worth knowing: this isn’t a standard flat slab. There’s a mini foundation element underneath what will be the first floor, that creates a weather tight utility storage area with an access point, basically a very functional crawl space.

It gives the homeowners somewhere to put mechanical systems and storage without eating into the living square footage.

It’s a small detail that makes a real difference, and it’s the kind of thing that comes from working with a contractor who’s built for multigenerational living before.

Why This Multigenerational ADU Is Built With ICF and What That Actually Means

ICF stands for insulated concrete form.

Instead of traditional wood framing, the walls are built from hollow foam blocks (think oversized interlocking Legos) reinforced with steel rebar and filled with concrete.

When it cures, you have a wall that’s solid concrete on the inside with foam insulation built right into both faces.

It’s a different way to build and considered an upgrade from traditional wood framing, and for this particular family and this particular use, it made a lot of sense.

When you’re building something that needs to be warm in a New England winter, quiet when the grandkids are running around outside and structurally solid for decades, ICF checks a lot of boxes at once.

What ICF brings to a multigenerational ADU specifically:

• Energy efficiency that keeps heating and cooling costs low year after year – The foam insulation runs continuously through the wall with no gaps, which matters a lot in Massachusetts winters
• Noise reduction – Concrete walls are naturally quieter, which is a real quality-of-life thing when you’re retired and living near an active family household with two kids
• Fire resistance built into the structure
• Durability that genuinely outlasts most of what gets built today

This is what I call Pre-Crisis Infrastructure, making decisions based on the life you’re heading toward, not just the one you’re in right now.

These parents are healthy and active. But they’re also building something that’s going to be their home as they age.

Building it right the first time is the whole point.

Jim has been doing ICF construction for years and it’s one of the things his company specializes in. Watching his crew work with it is genuinely impressive, there’s a precision and a rhythm to it that you don’t get with every build.

Building the ADU Walls: The Part That Looks Like a Giant Puzzle (Because It Kind of Is)

Once the slab was done, the crew started stacking. ICF blocks go up course by course in an offset pattern (like bricks) and every window and door opening has to be framed before the blocks get too high.

This part is important: you cannot cut through cured concrete after the pour.

Jim has done it, added a window after the fact, jackhammered through a finished wall. His description: a nightmare! 🤪

So the window and door frames get built first, secured into the ICF blocks, and then the blocks go up around them.

Once the concrete is poured, those frames are a permanent part of the wall structure.

I love watching this stage of a build because it’s the moment the floor plan starts to become a real space. You can stand inside the block perimeter, look through where the windows are going to be, and start to picture it.

What Holds 9-Foot Concrete Walls in Place?

Before any concrete goes in, the walls have to be braced.

And when I say braced, I mean seriously braced, you’re about to pump an enormous amount of heavy wet concrete into those foam forms, and those forms need to hold their shape while that happens.

The orange metal system you see in the photos anchors from the face of the wall down to the subfloor, holding every course perfectly straight and plumb.

It also doubles as the staging platform, the walkway the crew works from during the pour. Jim’s crew uses a dedicated ICF bracing system designed specifically for this kind of build.

The prep work takes days before the concrete pour days, getting the wall perfectly straight, level and braced in place.

Standing inside the braced walls before the pour was one of those moments where I understood exactly why this process takes the time it takes. Everything has to be right before that concrete moves.

Concrete Pour Day on a Multigenerational ADU Build: What It Actually Looks Like

Pour day has energy.

There’s a concrete mixer truck, a pump truck with a boom arm that extends up and over the building, and a full crew that needs to work in sync because once you start, you finish. You cannot stop mid-pour. Like, no bathroom breaks. No phone breaks. You are dialed in or Jim will you lose his mind 🤪.

The way it works: the mixer truck feeds into the pump truck, the pump truck extends its boom over the building, and a hose drops down into the ICF wall cavities. (same process as the twin brother’s multigenerational compound completed last year)

Jim is up on the staging directing the hose, walking the perimeter of the 30×30 structure, filling the walls from bottom to top with concrete.

I climbed up there to get the overhead shots and honestly I’m glad I did. The view from the top of a wall you’re about to fill with concrete is something to be seen!

That’s Jim. He’s done enough ICF pours that he moves through it with a kind of focused calm.

The One Step Nobody Thinks About (But Every ICF Crew Takes Seriously)

Right behind the person filling the walls, there’s a second person with a concrete vibrator.

It’s exactly what it sounds like, a long flexible wand that vibrates at high frequency, and you work it into the concrete from top to bottom of every wall cavity as the pour progresses.

The reason: air bubbles. When you pour concrete into a narrow cavity, air gets trapped in there.

Left alone, those air pockets become weak spots in the wall down the road.

The vibrator moves the concrete around every piece of rebar, pushes out the air, and makes sure the wall is solid all the way through.

In the tighter spots (near the window sill areas especially) the crew also drilled down through the subfloor from above and ran the vibrator down from the top to make sure nothing got missed near the base of the walls.

It adds time to the pour but it’s an absolute necessary step.

What a Multigenerational ADU Compound Looks Like From 50 Feet Away

Step back from the site on pour day and here’s the picture: a big red concrete mixer truck, a pump truck with a boom arm reaching over the building, Jim’s crew working from the staging, and this compact white structure in the middle of a cleared lot.

It’s a pretty cool site, right?

It’s a lot of machinery for what’s going to become someone’s quiet home.

It might not look it right now, but the space is generous. It might not look like much now but wait until you see the after pictures….it won’t even look anything like this.

What’s Coming Next in the Multigenerational Compound Series

The walls are poured and cured. Next post: the roof structure and interior framing (which is when 900 square feet stops looking like a concrete box and starts looking like someone’s home).

Coming up in the series:

• ADU roof structure and interior framing
• Main house foundation and wall construction
• Compound design: privacy, sightlines, shared utilities, all of it
• The planning conversations this family had before the build started
• Real cost data as the build progresses
• Both interiors as they come together

If you’re planning a multigenerational build, an ADU, or just seriously thinking about whether this is something your family could pull off, this series is for you. Not the brochure version. The real version.

Related: Massachusetts ADU Law: What Changed and What It Means for Your Property

Ready to Start Planning Your Own Multigenerational Space?

The family in this series did something that made everything easier: they had all the hard conversations before the build started.

Privacy, money, shared spaces, long-term care, what happens if things change, all of it, on the table, before a single permit was pulled.

Most families skip that part. And then they figure it out the hard way.

If you’re in the planning stage, start with the free Multigenerational Compound Planning Checklist, 96 questions to work through before you build, buy, or renovate.

Grab the free Multigenerational Compound Planning Checklist

And if you’re ready to go deeper, into floor plans, privacy design, shared systems, budget strategy, and the design decisions that make multigenerational living actually work, the Multigenerational Compound Planning Guide covers all of it which is coming soon, April 2026! If you grab the freebie above, you will be sent a link automatically once it comes out.