How to Add a Window Sill Extender

Man holding a level up to a window sill
  • 2-3 hours
  • Beginner
  • 50-150
What You'll Need
Wood for extension
Small dowels
Measuring tape
Wood glue
L shaped wall brackets
Primer and paint (optional)

Whether you need a place to set your plants or a seating area for a pet, you might need to add a window sill extender to get the space you need. You can replace your entire window sill to get the space you want, or you can simply extend the window sill you already have.

Step 1 - Finishing the Current Window Sill

Use a saw and cut the window sill flush with the wall on either side of the window. Use sandpaper to make sure the edge is flush and that it is smooth as well. You can choose not to do this step, but there will be a distinct groove where one piece of wood ends and the other begins. If you saw the wood, the extender will look more natural.

Step 2 - Make Measurements for Window Sill Extender

The window sill extender can come out of the wall as much as you want it to, but the further it comes from the wall, the more difficult it will be to support any weight on it. The length of the wood should be the same as the current window sill so it will attach easier and remain sturdier. Cut your piece of wood to the desired size and proper length.

Step 3 - Prepare the Wood

The piece of wood and the current window sill will be attached using dowels and wood glue and further supported by wall brackets. You will be drilling holes in both the sawed edge of the window sill and the side of the wood that will attach to it. Holes should be drilled every 3 to 6-inches. Make sure you drill holes so that the window sill and piece of wood will line up properly. Measure the distance with a tape measure. Mark potential holes with a pencil and make sure they line up before you begin drilling. Drill the holes big enough for the dowels to fit snugly inside.

Step 4 - Attach the Wood Pieces

Using wood glue in the holes you drilled attach the dowels to the window sill. Following the wood glue directions attach the window sill to the piece of wood using the dowels.

Step 5 - Attach the Wall Brackets

The size of L-shaped wall brackets you use will depend on how big the window is and how big the extender is. Use your own discretion, but if you have a bigger window or a big extender you might even want to use 3 "L" brackets, one on either side and one in the center of the extender. Attach the "L" brackets to both the wall and the underside of your window extender using screws.

Step 6 - Finish and Paint

If you want, use a wood stain or a paint that matches the trim to finish your window sill extender. Sand down any rough edges on the window sill extender before you paint.