I'm planning on building a built-in window seat against a wall that has two windows with sills and an apron that extends from the wall about 3/4" to 1":

Photo of window

What's the best strategy for removing this extended part of the sill so that we can build flush against the wall?

Figure we have the following options:

  1. Replace with new sill
  2. Cut existing sill flush with wall
  3. Build on top of or around existing protrusion

Doing #2 would clearly be the easiest but I'm concerned about any draftiness and sealing it back up right. I'd prefer not to do #3 as it would affect the height of the seat by a few inches.

Any suggestions are appreciated.

For reference, here are our renderings of the intended unit. The gray part is a vent for the radiator behind:

Builtin Builtin Detail


2 Answers 2


I would agree with #2. Sealing it up would be easy with some spray foam or caulking. You would have to put some type of flush trim on to hide the cut, or chisel out a bit more of the sill and piece in some sheetrock and mud it and/or change the window trim to a picture frame casing. Not really a hard job, just not sure what you want. I would like to see what you want the seat to look like in relationship to the window. How much lower than the window do you want the top of the seat to be?

  • The builtin will be paint-grade ply painted white, with the top seat made from some vintage lumber. The intention was to have the top flush with the sill so that it makes for one continuous surface, meaning whatever patching done to the wall below the sill will be covered by the unit anyway. Picture frame casing might be a nice touch.
    – Amit
    Feb 25, 2011 at 13:04
  • Added the drawings so you can see what's up. For reference, that's a vent for the radiator that you see in gray between the windows and seat. We might be able to finish / patch the edge of the sill with the framing for the vent...
    – Amit
    Feb 25, 2011 at 13:16

I'm going to suggest that you'll get your best results going with option #1. If you remove the stools you'll be able to scribe the rear fixed portion of the bench top to fit inside the window openings in place of the original stools, making it one continuous piece. This is pretty much what your rendering shows and I think this is what makes for the best-looking window seat construction. It's going to be more work as you might need to remove the window jambs to get the stools out.

You could possibly simulate that single piece look by cutting the existing stools flush to the wall as you suggest in option #2. But I think it's going to be tricky to make that cut smooth and straight with the stools still in place, and if it's not a nice cut you're going to have a jagged seam to fill there. At any rate if you go with this option I'd use a biscuit joiner to positively key the two pieces together to make sure they stay flush.

EDIT: Oops, just noticed that part about the radiator vent. So the fixed rear piece is mostly going to be open for the vent. I think this answer only applies if you decide to make that vent opening smaller (i.e. small enough that it makes sense to make that rear piece out of a single board).

  • Thanks for the advice. Agreed that the cut seems tricky and a wholesale replacement of the stools would give the opportunity for doing a nice clean job. We're actually having a carpenter do the build so not sure of his comfort in doing that work or what it'll do to our budget but will definitely make the suggestion.
    – Amit
    Feb 26, 2011 at 3:39
  • 1
    If the sill extends under the window frame itself, getting it out could be a problem. A good carpenter will be able to make a good cut with a sonic saw. Scribe a line at the desired joint location, make the cut apx 1/16 to 1/8th inch out from the line, then using a plane or sander, finish to the straight line. This is the same technique as scribing any other custom fit piece, counter etc. Clamping a straight guide piece to the sill before sanding will help assure a straight finish. Feb 26, 2011 at 12:28
  • @shirlock - I should have made clear that my answer was based on my experience doing something similar with vinyl windows where the stool butts up to the inside surface of the window frame. I don't think I'd try to remove it if it extended under (or was part of) the window frame. Feb 28, 2011 at 19:08

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