I am always shocked by the difference trimming out a window makes. Like, every time. I never notice how boring my windows look until I start adding trim, then I want to go around trimming out every window in the house! Not only does it make them look more custom, but it also makes them look so much bigger. And it’s such a great beginner project, it shouldn’t take you very long to do even if you don’t have a whole lot of experience.

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*note – the length of your boards will be dependent on your specific window, so you may be able to get both side pieces out of one 1x4x8 or you may need two 1x4x6s. This is what worked for my windows.


Start by taking out your window sill and the existing apron, and any other trim that might be there. Be careful when taking off your sill not to gouge your frame. It’s not something you need to be overly delicate with, but it’s worth paying a little extra attention. You can start with the 5-in-1 tool and then move to the crowbar.

Next comes the trickiest part, creating your new sill. Take your old sill and place it on your 1×8 (some windows may allow for a 1×6, just make sure you have enough extra depth to account for the additional thickness of the 1x4s you’ll be adding). Line the edge that sits against the window up to the long edge of your board, then move your board in from the short edge. Since you’re adding 1x4s on the edge, you need the “wings” of the sill to be extended by the width of those boards. So from the short edge, move the board in 3 5/8″ (3 1/2″ for the 1×4 and then an additional 1/8″ for the reveal).

Then on the other side, draw a straight line using your speed square 3 5/8″ away from the edge of the sill. Using the old sill as a template, trace the cut out for the window onto your new sill and continue the straight line to the edge of your board. Now cut the board to length with your miter saw and use the jig saw to cut out the square.

Nail the sill into the framing using the nail gun.

Measure the distance from the top of the sill to the top edge of the window box and add 1/8″ for the reveal. Measure on both sides to make sure they’re even. If they aren’t, try to at least make sure that the top is even with the ceiling. Cut the 2 of the 1x4s down to length.

Place then 1/8″ in from the edge of the window box to leave a 1/8″ reveal. If you prefer, you can make them flush, I just find it is a cleaner finish to caulk the corner created by the reveal rather than trying to fill and make the line between the box and the trim disappear.

Make sure your board is level and use the nail gun to nail both sides up.

For the top header piece, measure the distance from the outside edge to outside edge of the 1x4s you just installed and add 1″ (if you have door casings with an overhang, match the overhang – so my doors have about 1/2″ overhang on each side, so I added about 1″ so that it would match). Then cut the 1×6 down to that size, center it on the window, and nail it up.

To add the apron, measure from outside edge to outside edge of the side 1x4s again and cut your last 1×4 to that length. Then line it up and nail it on.

Finish everything by caulking and painting (both of which I still have to do…) and violá! You did it.

The total for all materials (tools not included) for two windows in my son’s room was $102, so for $51/window this is such a cost effective and easy update to make! Let me know if you try it!

With love,
Mercedes ♥

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Are you wanting to tackle this yourself? Here are links to some of my favorite tools used in this project.


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