Corner Window Trim — Casing Two Windows That Meet at a Corner
Two windows meet at the corner of the room and there is nothing between them but a post. The trim has to wrap that corner and make both windows read as one, and most trim carpenters have never done one.
Corner window trim is for two windows that meet at an inside or outside corner with no wall between them, only a post or a mulled frame. JMB Builders trims corner windows across the Twin Cities metro: jamb extensions run to the corner, a stool mitered around it as one piece, an apron wrapped below, a post cover where the two windows meet, and standard casing on the outer legs. Priced per window, quoted from photos within one business day.
- Priced per window; the corner pair counts as two
- Stool and apron wrapped around the corner in one piece
- Corner post covered, never cut or moved
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
A corner window is two windows set so close to the corner of a room that there is no wall between them. On the inside you see glass, a post or a narrow mulled frame, and glass again. Sometimes it is an outside corner instead: the windows sit on a bump-out and the corner points into the room. Either way there is no place for two full sets of casing to land, and trimming each window on its own leaves two legs fighting for the same few inches at the corner.
The parts that matter are the stool, the apron and the corner itself. The stool has to turn the corner as one piece, mitered at the angle, so there is no joint where the two windows meet and no place for the two halves to move apart. The apron under it wraps the same way. At the corner, a post gets a cover: three or four flat boards wrapped around it so it reads like one thick leg shared by both windows. A mulled frame with no post gets a single narrow corner casing instead of two. The outer legs and the heads are then normal casing, run to the corner and stopped against the corner piece.
One thing to know before you call: the post in a corner window is usually holding up the wall above it. JMB Builders covers it and trims around it. We do not cut it, move it or take it out. If you want the post gone for a glass-to-glass corner, that is a structural change with an engineer and a permit, and we refer it out. JMB Builders trims corner windows across the Twin Cities metro on new builds, replacement windows and remodels. Painting is quoted as a separate line, or leave it primed and caulked and paint it yourself.
What you supply
- A photo of the corner from inside, straight on to each window, and a close-up of what is between them
- Whether the corner is an inside corner (windows on two walls of the room) or an outside corner (a bump-out)
- Your casing profile, or a photo of another trimmed window in the house to match
- Paint or stain color if you want us to finish it; painting is quoted as a separate line, or paint it yourself
What JMB supplies
- Jamb extensions cut to run into the corner on both windows
- A 5/4 stool mitered around the corner as one piece, with horns on the outer ends only
- An apron wrapped around the corner under the stool, mitered to match
- A post cover from flat stock, or a single corner casing on a mulled frame with no post
- Casing on the outer legs and across both heads, matched to the rest of the house
- Miters glued and pinned across the joint so the corner stays closed through the heating season
- Nail holes filled, joints caulked, ready for paint; painting or staining quoted as its own line
- Photos of the finished corner in your job folder
How it is priced: per window
Quoted in writing from your photos within one business day. What moves the number:
- Post between the windows vs. a mulled frame with no post; the post cover is more pieces
- Inside corner vs. outside corner; the outside corner stool has a longer miter and is more exposed
- Whether the windows need jamb extensions or already sit flush with the wall
- Stool and apron vs. a picture-frame wrap with no stool
- Paint-grade vs. stain-grade, which is cut tighter at the corner miter
- Matching an existing casing profile vs. a standard one
A photo taken from the middle of the room showing both windows and the corner, plus a close-up of the post, is enough to quote it firm.
How long it takes
One corner pair with finished jambs: most of a day. With jamb extensions on both windows: a full day. A room with two corners: two days.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Corner with a post between the windows
The post gets wrapped with flat stock so it reads as one thick shared leg. The stool and apron miter around it, and the outer legs and heads are normal casing. The most common corner window on new builds around the metro.
Mulled corner with no post
The two window frames are joined at the corner by the manufacturer with a narrow mull. A single corner casing covers the mull on the inside, and the stool wraps as one piece under both windows. Nothing structural is touched.
Outside corner on a bump-out
The windows sit on a bump-out and the corner points into the room. The stool miters around the outside corner with a longer exposed joint, so it is glued and pinned across the miter and the horns are returned at the ends.
Drywall-returned corner window
Keep the drywall returns on the jambs and heads and add only a wrapped stool and apron at the corner, so there is a ledge and the corner does not chip. The lowest-cost version of a corner trim-out.
Corner Window Trim — Casing Two Windows That Meet at a Corner: questions, answered
How do you trim two windows that meet at a corner with no wall between them?
The stool is cut as one piece and mitered around the corner, so both windows share it. The apron wraps under it the same way. The post or mull at the corner gets a single cover or corner casing instead of two casing legs, and the outer legs and heads are standard casing run into it. Done that way the two windows read as one unit instead of two that collided.
Can you remove the post between corner windows so the glass meets?
No. In most houses the post in a corner window is carrying the wall and roof above it. Taking it out is a structural change that needs an engineer and a permit, and it usually means a new header and a different window unit. We cover the post and trim around it; if you want it gone we refer you to an engineer first.
Why does the trim at my corner window have gaps at the corner?
Because the stool or the casing was cut in two pieces and just nailed at the corner. The two halves move separately every winter and the joint opens. A corner stool that is one piece, or two pieces glued and pinned across the miter, stays closed. We pull the split trim, check the framing behind it is flat, and rebuild the corner as one piece.
Is a corner window priced as one window or two?
Two windows, with the corner work in the price. Each window gets its own jamb extensions, outer leg and head, and the shared stool, apron and corner cover are split between them. A room with two corner pairs is four windows. The quote lists it that way so there are no surprises.
Can a corner window be trimmed in picture-frame style without a sill?
Yes. The casing wraps all four sides of each window and the bottom pieces miter around the corner instead of a stool and apron. It is cleaner on a modern house with picture-frame trim on the doors. The corner post still gets a cover, and the bottom miter at the corner is glued and pinned like any other.
Do you trim corner windows on a new build where the builder left drywall returns?
Yes. That is where most corner window requests come from. The drywall returns can stay on the jambs and heads with just a wrapped stool and apron added at the corner, or the whole pair can get jamb extensions and full casing. Send a photo from the middle of the room showing both windows and we quote both ways.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and for the builders and dealers we sub for. Counts come straight from our job log; no customer names.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 26, 2026 · JMB Builders LLC, MN license BC809005 · All window trim pages
Where we work
Window trim near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: