The Sidewalk That Can Kill Your Infill Deal: Infrastructure Costs to Know Before You Buy
The short answer
Zoning tells you what you can build. The city and highway district tell you what you have to build first. In Boise, that can mean widening your half of the street, adding a detached sidewalk with a landscape buffer, engineering a drainage plan, rerouting irrigation, proving sewer access and giving fire and trash trucks a place to turn around. Miss those costs and a six-figure profit can become a six-figure mistake. Account for them up front and a deal like this week's Southwest Boise lot split still pencils.
You can buy a great lot at a great price and still lose money
You can find a property zoned for three homes, buy it at a great price, and still lose money on the deal. And it might have nothing to do with the purchase price, construction costs or what the homes sell for.
It could be something as simple as a sidewalk.
I see this over and over with first-time developers, especially people coming from flipping houses or owning rentals. They assume that because zoning allows a split, the deal pencils. What they don't know is what the city can require them to build before those lots can be developed.
In infill development, it's not only what you can build that decides whether a deal is profitable. It's what you have to build.
The infrastructure costs that catch investors off guard
Every jurisdiction is different, so you have to know the rules where you're buying. Here's what Boise and the Ada County Highway District (ACHD) typically require on a lot split.
Road widening. ACHD requires the roads serving your property to meet modern standards. Most infill lots sit on streets built 50 or more years ago. On this week's deal, the street is 27 feet wide and the standard is now 33 feet. You don't rebuild the whole street. You widen your half, from the centerline to your property line, across the width of your lot.
Detached sidewalks. There isn't a single sidewalk on this street today. The city will still require one, and it's no longer a sidewalk attached to the curb. It's an 8-foot landscape buffer, usually with one or two street trees, plus a 5-foot sidewalk. Busier street classifications can require wider sidewalks, buffers or setbacks.
Curb, gutter and frontage landscaping. These come along with the sidewalk and have to meet ACHD and city standards.
Sewer access. Knowing "sewer is in the street" isn't enough. You need to know where it is, how deep it is and what it costs to reach. If sewer is 500 feet down the road, this deal doesn't pencil. At 100 feet it might, or might not. Sometimes the fix is an easement across the existing lot, which you already own, to serve the rear lots.
Drainage plans. Large subdivisions have always had to keep stormwater on site. In the last year, Boise has started requiring engineered drainage plans on small splits too, after neighbors complained about runoff from new homes. That's an engineering cost and a construction cost most people never budget.
Irrigation. Under Idaho law you can't cut off irrigation to downstream users. If a ditch or line runs through the lot where a house will sit, it has to be rerouted, and that has to be engineered and built.
Fire and trash truck access. If a street dead-ends without a turnaround, fire and sanitation will flag it. That has to be mitigated, sometimes temporarily until the rest of the street develops.
Additional requirements. There are at least a dozen other requirements from different agencies that can pop op on any given deal. Utilities, easements, hillside, street lights, fire hydrants, just to name a few.
Time. Every one of these adds review time, and time is a cost too.
Why "just ask the city" isn't due diligence
City staff are helpful within what they know. But when you walk in or call, you usually get the newest planner at the front desk. Many associate planners cover one slice of the code and may not know how public works operates or think to send you there.
If sewer isn't available, you'll likely hear "you can't split that." You won't hear about the sewer easement option. That's how an investor who bought without full due diligence ends up calling a workable deal dead in the water.
Our due diligence runs a comprehensive checklist on every property. We look for yellow and red flags, and we put a cost on every one of them before you buy, not after.
This week's deal: a double lot split on Penninger Drive, Southwest Boise
The property
Existing home: 4 bedrooms, 2 baths, a little over 2,100 SF with a basement, remodeled and turnkey
Lot: 75 feet wide, with a rare second public street along the back
Location: Southwest Boise, close to the freeway and the shopping around the Spectrum movie theater area
The plan
Because the back of the lot fronts a public street, we can split two lots off the rear and keep the existing home. Each new lot is 37.5 feet wide, which is 12.5 feet wider than a lot of Boise row home lots. We build two new homes.
The infrastructure we already priced in
Widen the front street 3 feet on our half, plus landscape buffer and 5-foot detached sidewalk
The rear street is narrow and serves only a couple of homes, so we budgeted the most road widening likely to be required, plus buffer and sidewalk
Drainage, curb and gutter, staking and frontage landscaping
A fire and trash truck turnaround, since the rear street dead-ends where the paving stops
Two yellow flags most people would miss
Airport influence area. The airport is a few miles away, and the county assessor's site doesn't show that this property sits in its influence area. That area adds requirements.
The sound contour. Inside the airport area there's a mapped noise zone that limits density. This property appears to sit just outside it, which is what allows the extra lots. A block over, it could kill the project. We'd confirm this during due diligence.
Get the numbers
The full numbers on this deal (purchase price, cash needed, projected profit, timeline and how the lender funds it) are in this week's Deal of the Week email. Every week it goes out with a one-page deal sheet and the full investor proforma, so you can run the math yourself.
Then join us live every Friday at 2 PM Mountain Time on the "Find Your Next Six-Figure Infill Deal" Zoom call. Each week we dive deep on one part of infill development and walk through a new deal line by line. Bring your questions.
Owner-occupants can put as little as 5% down on the existing home with a traditional mortgage, live in it, and still build on the rear lots because they become separate properties.
What happens if costs go up or prices go down
No deal is risk-free, so the investor proforma includes a sensitivity table. It shows what happens to profit if construction costs rise or sale prices fall, and how far prices can drop before the deal loses money. We underwrite conservatively, with contingency and construction inflation built in, because our goal is to under-promise and over-deliver.
On top of that, our Profit Guarantee adds a backstop when Matson Homes is hired for every phase and the project reaches Certificate of Occupancy. It's subject to the Profit Guarantee Agreement.
The bigger play: 9 doors as a rental
Boise's ADU rules now allow up to two ADUs on a lot, as long as they fit. The existing home has a basement that could become one ADU, with room for another on the lot. Each new lot could also carry two ADUs: basement, attached, detached or two-story.
That turns one property into 3 lots and up to 9 doors. You can phase it: build the two ADUs on the existing lot while the split goes through entitlements, then one new home and its ADUs, and so on. If you want the rental version underwritten, I can turn around a proforma, with an investor side and a lender side, in about 30 minutes.
How it works when you buy a deal from us
You buy the property and own it. You hire Matson Homes for due diligence, entitlements, site development and construction. You make the big decisions along the way, we lay out the options, and you can exit at any stage. We prefer clients who go all the way through construction, which is why our guarantees reward finishing.
Other deals on the list this week
Cloverdale cottage village: 9 cottages plus the existing home. One neighboring lot went pending this week; this one is still active.
Victory Road (two properties): about 3 acres combined, room for 38 cottages plus the 2 existing homes, with savings from developing both together.
North End cottage village: still in play.
45th Street vacant land: still in play.
17 build-ready townhomes: utilities on site and permit-ready, buildable in phases, for investors who want a shorter hold.
The numbers on each are available to our Deal List Members.
Join us live
We go live every Friday at 2 PM Mountain Time on the "Find Your Next Six-Figure Infill Deal" call. Bring your questions; I stay after to talk through your situation. Can't make it live? Subscribe on YouTube for the replay, join the email list for the one-page deal sheet, and send me a message on Instagram @thatflippinguy
Projections are based on today's comps and preliminary underwriting. They are not guarantees of future results. Guarantee terms are preliminary and subject to the Profit Guarantee Agreement.
What infrastructure does Boise require when you split a lot? Typically: widening your half of the street to current ACHD standards, a detached sidewalk (an 8-foot landscape buffer plus a 5-foot sidewalk on a local street), curb and gutter, an engineered drainage plan that keeps runoff on site, sewer and water service to each lot, irrigation rerouting if a line crosses a building site, and fire and trash truck access.
Do I have to widen the whole street? No. ACHD requires you to widen your half of the street, from the centerline to your property line, across the width of your frontage.
Does Boise require drainage plans on small lot splits? Yes. Within the last year, Boise has started requiring engineered drainage plans on small splits, not only large subdivisions. Roof runoff and stormwater must stay on site and can't drain into the street or onto neighbors.
What if there's no sewer in front of the new lots? Distance decides the deal. A sewer extension of about 500 feet would kill this week's deal; about 100 feet might work. A sewer easement across the existing lot you own is sometimes an alternative.
What is the airport influence area in Boise? It's a zone around the Boise Airport with added development rules. The county assessor's site doesn't show it. Within it, a mapped noise contour can limit density, so a lot just outside the contour may allow more units than one a block away.
How many ADUs can I build in Boise? Boise's current rules allow up to two ADUs on a lot, as long as they fit the site and meet code. On a three-lot split, that can mean up to 9 doors.
What's the plan for the Penninger Drive lot split? The plan keeps the existing home and builds two new homes on the rear lots over about three years. The full numbers are in the weekly Deal of the Week email (one-page deal sheet and proforma) and on the live Friday 2 PM Mountain Time Zoom call.
