How to Replace an Existing Window With a Garden Window

By thewriteDuffy •  Updated: 04/08/24 •  7 min read

Since researching and writing about all the many types of windows, I’ve become obsessed with garden windows.

After sharing my obsession, I’ve managed to score the chance to sit in on the replacement process, so here it is!

Let’s walk step-by-step through how to replace an existing window with a garden window.

Tools Needed for Window Replacement

Materials Needed for Window Replacement (for Garden Window Upgrade)

Steps to Replace an Existing Window With a Garden Window

Step One: Measure, Order, and Plan Your Window Replacement

You’ll want to start at the very beginning by measuring the existing window you’re hoping to replace. Determine the size of the window by estimating the rough opening — take the measurement from the inside extension jamb on one side to the inside of the extension jamb on the other side and add two inches.

Once you have the size, it’s time to shop around to order your new garden window. You’ll need to plan enough time to order the window. The units can take several weeks to manufacture and deliver.

Some companies custom-make garden windows to fit the exact size you need. Other companies have stock sizes and you will need to order dimensions that are close to your existing rough opening, which you’ll then frame to fit later on.

Recommendation: I recommend getting a garden window that’s an all-vinyl window with energy-efficient insulated glass. Garden windows come in wood and other forms, but vinyl is the most popular choice because it stands up to weather well and lasts longer.

Don’t forget to make sure you shop for all the tools you’ll need as well (there’s a handy list at the top of the post) including a reciprocating saw and a miter saw. Doing this job without the right tools will make things very difficult and possibly lead to costly mistakes!

Pro Tip: Check with your local municipality to see if a permit is needed for your new garden window since it does protrude from the side of the house.

Step Two: Remove the Old Window

When the window arrives, it’s time to get to work!

Check the weather for a nice day (you’ll have a hole for a good few hours during this project, so you don’t want to pick a bad day), and gather up all the tools and materials you’ll need (there a handy list at the top of this post for you!).

When you’re ready to start, go to the outside of the house first and use a utility knife or chisel to cut away the caulk and sealant around the window to be removed

Inside, use your knife to score the edges of the casing all around the window, which will prevent the paint and drywall paper from pulling away when you remove this trim. Next, use a pry bar to lift and remove the interior trim and casing.

Pro Tip: As much as I hate to admit it, saving the trim on this job won’t work. It’s going to be easier (and often necessary) to just replace it.

With the trim removed, along with any insulation tucked in the gap between the window and house framing, you’ll see the nails or screws that hold the window in place. Use a reciprocating saw or hacksaw with a metal-cutting blade to cut all of these free.

Remove the window by having one person push from the inside while one or two helpers on the outside catch the window as it comes free.

Even small windows are heavy, so be cautious and be prepared. If the window sill height is low enough, set up sawhorses or a scaffold to “catch” the window onto as it comes out. Broken glass will just delay your whole project.

Step Three: Inspect the Area and Replace Any Rotten Wood

Chances are that if you’re replacing a window, the existing window has seen better days. If this is the case, and especially if the old window had a wooden frame. Inspect the window frame and surrounding area for any rot from water leakage.

If you see anything concerning, pry off a bit of the siding carefully to see if there is any rot and if so, how far the rot extends.

The last thing you want to do when installing a new garden window is to cover up a bunch of rotten wood, so anything that is bad will need to be replaced.

If you’re not doing the work yourself, this is the kind of thing to watch out for when you hire people to do work for you. Hang around and take a look when they get the old window off.

Step Four: Install the New Garden Window

Before installing the new garden window, find and mark the center of the window opening. Also determine the installation height of the garden window, which may be different from the old window and the rough opening, and take the time to mark a level line across the opening.

If the rough opening is taller than you need, it’s generally better to install the new window to the top of the opening and fill in the space below it. This will help to maintain the home exterior’s defense against rain and water runoff, and any siding replacement will be less noticeable below the window than above it.

If necessary, add spacer blocks to the rough opening sill to elevate the new window up to the level line.

Install an aluminum drip cap above the window opening even if the manufacturer claims the garden window does not require it.

Tuck the drip cap underneath the siding so the drip edge extends out over the top of the window.

Lift the garden window up to the scaffolding and place it into the opening. Ensure that the window is level, vertically plumb and centered in the opening.

Insert tapered wood shims between the window and house framing to firmly position the window without forcing or bowing in the window sides.

Double-check the window alignment in all dimensions, then screw through the window sides and shims into the house framing.

Step Five: Trim the Window Inside and Out

On the outside of the window, attach mounting brackets provided by the manufacturer and fasten them to the house sheathing or framing for added support.

Cover up any gap in the house sheathing below the window and install siding to match the original.

Measure, cut, and install the garden window’s exterior trim according to the manufacturer’s instructions.

Caulk around the window to complete the work on the outside and seal the window against the elements.

Inside, fill the space between the window frame and rough opening with insulation to prevent drafts.

Measure, cut, and attach the interior casing and trim with finishing nails.

Apply paintable caulk to the seams and nail holes around the trim. Be sure to fill the gaps thoroughly to ensure good weatherproofing.

Install any clips or brackets provided to hold the garden window’s glass shelf.

Step Six: Enjoy Your New Garden Window!

Once all the above is complete, it’s time to get out your herbs and houseplants or decor and enjoy your new garden window view!


At home, April is a mom, wife, and DIY darling. Among other home projects, she helped her husband Dan renovate their 1986 bungalow and is currently designing and decorating the 2023 custom home they are building themselves. Professionally, April is a writer, author, and online marketer with 15 years of experience writing for newspapers and magazines, building online authority websites, and publishing books.