Window Seat Dimensions

Published window seat depths run from 16 inches to 30. That looks like disagreement. It is actually two different pieces of furniture wearing the same name.

A 16 to 18 inch window seat is a perch. Feet on the floor, back straight, dining posture. You sit on it to put your shoes on or to eat breakfast.

A 20 to 24 inch window seat is a lounge. Legs up, back against a side wall or a pile of cushions, sideways. You sit in it for an hour with a book.

Decide which one you are building before you look at another number, because everything else follows from it.

Both versions are a box you sit on. You do not have to build either one, and a lift-top storage bench is the same object bought rather than made, which is covered on lift-top and drawer benches. Building it yourself buys you the exact window width, which is the one thing a bought bench cannot give you.

Depth

This Old House gives 16 to 20 inches and repeats the range in its build guide. Today’s Homeowner extends it, splitting by use: casual seating 16 to 18, reading nooks 20 to 24, bay windows 18 to 30, with the warning not to go under 18 inches if you want people to actually sit there.

Both are right. The narrow end is the perch, the wide end is the lounge.

One US standard does put numbers on a bench, and it lands in the same place. ADA 903.3 requires an accessible bench seat to be 20 inches deep minimum, 24 inches maximum, and at least 42 inches long. That is the lounge range. The standard exists for wheelchair transfer, so it arrives at those numbers from a completely different trade, and it was published by a federal agency instead of a decorating site.

One practical note on the lounge version. If you want to sit sideways with your legs up, the seat has to be long enough for your legs. Hip depth is a different measurement and it will not tell you whether they fit. A 24 inch deep seat that is only 36 inches long is still a perch, because there is nowhere for your feet to go.

Height

About 18 inches from the floor, including the cushion. This Old House says about 18. Today’s Homeowner gives 16 to 20 and describes it as roughly what you would expect from a dining chair, which is exactly the right mental model.

ADA 903.5 puts the top of an accessible bench seat between 17 and 19 inches above the floor, which brackets the 18 that the carpentry sources give. Eighteen sits in the middle. One number came out of building furniture and the other out of a federal accessibility standard.

Lower for children. Today’s Homeowner gives 14 to 16 inches for children’s bedrooms and hallway seats.

The same frame-versus-finished trap applies here as with bench seating. If a plan gives you a height, ask whether the cushion is in it. See bench and banquette dimensions for the compression arithmetic.

Length

This Old House gives an absolute range of 30 to 50 inches. Today’s Homeowner gives 3 to 8 feet, with bay windows at 4 to 8, and offers the only relative rule I could find anywhere: match or slightly exceed the window width.

That rule is worth following. A seat noticeably narrower than the window looks like a mistake. A seat that runs wall to wall under a window looks built in even when it is not.

The accessibility standard sets a floor rather than a target here. An accessible bench has to run at least 42 inches, which is about the shortest a window seat can be and still seat two people.

Cushion

Two to four inches. This Old House says 2 to 4 in both articles. Today’s Homeowner says plan for 3 inches minimum on top of the base.

Foam density, and how thick a cushion needs to be before a hard box stops feeling like a hard box, have their own page: window seat cushions.

Backrest, where there is one, runs 10 to 20 inches per This Old House. That is lower than the 16 inch minimum Fine Homebuilding sets for a dining banquette, and the reason is that a window seat back is often the wall, the window frame, or a stack of pillows rather than a built element.

ADA 903.4 asks back support on an accessible bench to start no more than 2 inches above the seat and reach at least 18 inches above it, which sits at the top of the This Old House range. It describes a built element you put your weight against. A window seat back is usually just the wall.

The bit nobody mentions

If the window seat goes in a bedroom, check the egress requirement first.

The International Residential Code, Section R310, requires emergency escape and rescue openings in sleeping rooms. The published minimums are a net clear opening of 5.7 square feet, a clear opening height of 24 inches, a clear opening width of 20 inches, and a maximum sill height of 44 inches measured from the finished floor. Grade-floor openings get 5 square feet.

Fall protection, if the sill is low

The section above covers R310, the egress rule. There is a second one. R312 deals with children falling out of windows, and a window seat interacts with it in a way the code never contemplated.

R312.2.1 applies where the bottom of a window’s clear opening sits less than 24 inches above the finished floor and more than 72 inches above the ground outside. Upstairs bedrooms clear 72 inches easily. Such a window has to do one of two things: refuse a 4-inch sphere at its widest opening, or carry an opening control device or fall prevention device meeting ASTM F2090.

Read the datum. Twenty-four inches above the finished floor. A seat is furniture, so building one does not change the code number, and a window that complied before the seat went in still complies after it. A child standing on the cushion is not reading the code. A sill sitting a legal 30 inches above the floor is 12 inches above an 18 inch seat, and 12 inches is a step.

If that window is also the bedroom’s escape opening, R312.2.2 says the control device has to release to the full clear opening R310 requires, so the two rules do not fight. None of this stops you building the seat. If the drop outside is more than about six feet, fit the control device before you fit the cushion.

Radiators, vents and moisture

Windows and heat sources live in the same place surprisingly often.

This Old House recommends lattice panels on the front of the seat to let heat escape, or a hinged top for radiator access, and leaving space between the window and the seat for air circulation to prevent moisture buildup. No dimension is given for that gap.

Today’s Homeowner does give clearances: 6 to 8 inches of unobstructed space behind vent grilles, at least 3 inches above radiators, and ventilation slots placed at least 4 inches above the floor.

The moisture point is the one that bites later. Cold glass, a warm room and a sealed box under the seat is a condensation problem waiting to happen. Ventilate the cavity.

Storage inside

If the seat lifts or has drawers, the usable interior is smaller than the outside by about an inch to an inch and a half in each direction, which is the carcass.

There is no published standard for this. The one manufacturer listing I could source gives a storage bench with exterior dimensions of 31.5 by 19 by 15.75 inches and an interior of 30.5 by 17.75 by 14.5. That is a loss of about 0.6 inches per side and 1.25 inches vertically.

So an 18 inch deep window seat gives you roughly 16 to 17 inches of usable interior. Treat that as arithmetic from one example rather than an industry rule, because it is not one.

This Old House’s build uses end cabinets at 12 inches deep by 24.5 tall by 30 wide, with intermediate cabinets 15 inches tall. Today’s Homeowner suggests at least 6 inches of height per drawer.

One number for whoever is building the box rather than buying it. ADA 903.6 asks an accessible bench to take 250 pounds applied at any single point on the seat, the fastener or the supporting structure. A lid on piano hinge over a 30 inch span will not do that without a rail under it.

Quick reference

Where to go next

Written by Franco N. Corner Nook publishes measurements with their sources so you can check them. See the storage benches these numbers describe.