Home Decor Home Improvement

How to Build a Window Box in 6 Easy Steps

6 min read

Now that spring has sprung, it’s the perfect time to satisfy that gardening itch and boost your curb appeal with a DIY window box. All you need is a beautiful window, a few tools and some free time. Read on to learn what kinds of tools you’ll need, how to build to build the box itself, and how to affix it to your window exterior. Scroll down to get started.

What is a Window Box?

Window boxes are decorative boxes that allow you to display flowers, plants, or other greenery. If you’re looking for a charming addition to your exterior space, window boxes are for you.

Gather Your Tools & Materials

What you’ll need for a DIY window box:


  • Pressure treated 2x4s or decorative wooden trim of your choice
  • ¾” thick cypress or cedar boards
  • 2 wooden brackets to attach box to your home
  • Rust-proof nails and wood screws
  • 1 quart of primer (if painting)
  • 1 quart paint or outdoor stain of your choice
  • Pea gravel or pebbles to cover the inside of the bottom of your box


  • Power drill
  • Hammer
  • Tape measure
  • Saw
  • Level
  • 2” paint brush
  • C-clamps (optional)

Let’s Start Building!

Time required: 2-4 hours active time, plus time for paint to dry

1. Measure Your Window

Start by using your tape measure to measure the width of the window you’ll be putting your box under. This will determine the length of most of your pieces of wood.

measuring window sill

2. Purchase and Cut Your Wood

Your box is going to be 8 inches tall, 11 inches deep, and the width of your window. All of the main pieces of wood will be 8 inches tall, while your trim will be shorter (exact measurements below).

Many home improvement stores sell 8 in planks of cedar or cypress, which makes cutting your pieces much easier. You can also purchase planks and cut them into 8 in wide pieces.

You’ll want to purchase enough cedar or cypress to cut the following pieces:

  • (3) 8 in wide x window length (these will be the bottom and long sides of your box)
  • (2) 8 in wide x 11 in long (these will be the short sides of your box)

And enough 2x4s or decorative trim to cut the following pieces:

  • (2) 1-¼ in wide x window length
  • (3) 1-¼ in wide x 5-¾ in long

Many home improvement stores sell 8 in planks of cedar or cypress, which makes cutting your pieces much easier. You can also purchase planks and cut them into 8 in wide pieces.

pieces of wood for window box

3. Assemble The Box

Start by laying one of the 8 in x window length pieces flat on your workspace. This will be the bottom of your box. Stand another piece of 8 in x window length along the long side of the bottom and secure with clamps (optional). Repeat this with the last 8 in x window length piece. This is the front and back of your box.

To attach them, drill pilot holes every 4-8” along the bottom edges with your ⅛” drill bit. Insert screws into the holes along front and back.

Repeat this process for the short sides of your box.

construction of window box

When you have finished assembling your box, be sure to drill drainage holes in the bottom with a ½ inch or ⅜ in drill bit. We recommend one hole every 6 inches.

4. Attach Your Trim

Now it’s time to add a decorative touch to your window box. With your hammer and rust-proof nails, attach one of the 1-¼ in wide x window length pieces along the top of the front of your box and the other along the bottom of the front. (You’ll only attach trim to the front of the box where it will be visible.)

When those pieces are secure, you’ll move on to the 1-¼ in wide x 5-¾ in long pieces. These will be placed vertically (perpendicular to the trim we just attached): one at either end of the front of the box and one in the middle.

attaching trim to window box

5. Prime and Paint or Stain

Your box is almost done! You just need to paint or stain it and attach it under your window.

If you choose to paint it, start with a primer to create a nice barrier to prevent the wood from soaking up the paint. Let the primer dry completely, then apply 1-2 coats of an exterior semi-gloss paint.

If you’re staining your box, apply the stain according to the manufacturer’s instructions.

Don’t forget to paint or stain your wooden brackets!

6. Attach Your Box Underneath Your Window

When your brackets and box are dry, it’s time to attach them to your home and fill them with flowers, herbs, or any other greenery you’d like to showcase!

  • Start by marking 1 inch below your window frame. If your window opens outward, you will need to measure further down to ensure your window can still open once the box is in place. Do this on each side.
  • Line up your bracket with the inside edge of your window frame and line up the top with the 1 inch mark. Create a pilot hole approximately 2 in from the top of the bracket, being sure to go all the way through the bracket to the side of your home. Then attach the bracket to the house with a 3-inch-long decking screw.
  • Use your level to make sure the bracket is perfectly vertical. When it is, drill another pilot hole an inch or so above the platform of your bracket and secure with a decking screw. Repeat this process for the other bracket. Use your level to make sure the brackets are even.
  • Place the box on the brackets (be sure the side with the trim is facing out!) and secure it with two wood screws through the back of the box: one in each bracket. You can also place screws through the bottom of the box into the brackets for extra security.
  • Put a layer of gravel or pebbles in the bottom of your box to ensure proper drainage. Place your plants inside and admire your handiwork!
complete window box

Complete Your Home Transformation With New Windows

Pair your window box with brand new windows to give your home’s exterior a complete makeover. Schedule your free estimate to learn about our windows and how they can transform your home.

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