Why Plan 4-Foot-Wide Stairways?

By Buz Artiano, BuildX | October 2025

A stairway takes up valuable space in an ADU. When we're trying to conserve every inch of space on a single floor, a four-foot-by-twelve-foot stair area can feel like it's taking a huge chunk out of the space. But we have acquired some knowledge along the way of building dozens of ADUs in Massachusetts that we would like to share. The extra twelve inches in width of the stairway is the difference between a home that adapts to your life and one that will later need a $10,000+ retrofit job later when you need it.

At BuildX, we've seen so many families tear apart brand-new staircases because they originally built them just to code minimum instead of what was the best option. We don't want this to happen to you. That's why we design every two-story ADU stairway at four feet wide, never three.

What Makes Four Feet the Right Width?

Code requires a minimum of a three-foot stairway, but we always recommend at least four feet so a chair lift fits and a caregiver can walk next to it as well. Even when folded, a chair lift is still in the way of a 3 foot stairway. At four feet, you have room to navigate safely without squeezing past equipment that's mounted to the wall.

Let's think about the three foot width for a second. If a chair lift takes up 16 inches when it's folded against the rail, that leaves 20 inches of walking space, which is adequate, but uncomfortable for anyone carrying groceries, laundry, or helping someone else up the stairs.

At four feet, you get 32 inches of walking space when the lift is folded. That's enough for two people to walk by and for furniture to be moved without damaging walls. But maybe most importantly is that it's plenty of room for EMTs to navigate with a stretcher in an emergency.

What Are the Technical Specs for Chair-Lift-Ready Stairs?

These dimensions will make sure you never need to go back and widen the stairway:

Add outlets during construction because it only costs about $150. After everything is closed up, you'll have to pay around $800 to $1,200 to get the same thing done.

What Does It Cost to Get This Right Versus Getting It Wrong?

Here's the math so you know why we recommend what we do:

Beyond the money, a retrofit means weeks of construction going on in your home, structural changes, and it adds a lot of extra stress to people who already have mobility challenges.

What If Site Conditions Limit Our Options?

There are times when we can't get the full 48 inches. When that happens, we work with what we have:

• 3.5-foot compromise: Acceptable for basic lifts when 48 inches is impossible
• Straight versus L-shaped stairs: Straight stairs are much easier for chair lifts. An L-shaped stair can work but it makes installation quite complicated.
• Platform or exterior lifts: These are options for when there is not enough room for a big enough stairway.
• First-floor primary suite: Create a design without stairs when possible
• Future elevator chase: Frame a closet stack now for potential vertical lift later

We need at least 13 feet of straight run to go down. If there are space constraints that force an L-configuration, we can design landings and storage for the lift and create clear loading areas at both ends.

Do Four-Foot Stairs Make Sense for Everyone?

Not every ADU even needs a stairway. If you'll never need a second floor, a single-story design eliminates the whole issue. And if you're building a smaller two-story unit where every inch counts, you might have to prioritize your living space now, rather than planning for future accessibility issues.

But we always tell people to remember that mobility needs change. A fall, a diagnosis, or simply aging can change someone's needs overnight. The families who planned ahead are always grateful they did. The people who didn't end up wishing they had spent the extra $1,200 during construction.

What Else Do Wider Stairs Improve?

Beyond chair lift compatibility, four-foot stairs give you other benefits daily:

• Much easier to move furniture with less damage
• Two-way foot traffic is safer for kids and pets
• EMTs can get up and down with stretchers in emergencies
• The overall feel of the stairway is more spacious, which helps resale value of the property

Wider stairs make everything feel larger and they improve the flow of the whole ADU. It's not just about accessibility, it's about daily living.

The Decision That Saves $13,800 Later

For less than the cost of an appliance upgrade, you can keep your independence in your home and avoid paying for a hugely expensive retrofit later. Trust us, just build the extra foot now.

If you're planning a two-story ADU in Massachusetts, download our ADU planning checklist to make sure your design accounts for future accessibility. When you're ready to discuss your project, Request a Free Consultation with BuildX and we'll walk through the specifics of your site.