How to Build Built-In Cabinets That Fit Any Space

There’s nothing better than built-in storage, especially when it’s integrated with the room’s architecture. When we finished our attic into a bonus room, we were excited to use the recessed niche around the window. In this post we’ll walk through how we designed and installed built-in cabinets using stock units — spoiler: it’s an Ikea hack.

This article focuses on the wall-to-wall built-in cabinets; we also documented how we added the DIY floating shelves, which complement the cabinets and create a cohesive feature wall. We separated the tutorials because each feature deserves its own step-by-step guide.

Step 1: Plan Your Built-Ins

The recessed portion of the room was only 17″ deep, so adding built-in storage made a lot of sense. At first the niche felt like a design challenge — it’s off-center, shallow, and framed by two sloped ceilings — but its narrow depth actually inspired the idea. We wanted storage that added visual height and interest to that wall while staying proportional to the window.

We used Ikea’s kitchen planner to experiment with cabinet layouts, trying several configurations and mockup shelf ideas. The planning tool made it easy to visualize proportions and create a final shopping list for the pieces we needed.

We settled on four standard 30″ tall base cabinets across the bottom, with a centered 20″ drawer base beneath the window. That layout created a pleasing “window seat” effect and felt balanced even though the niche itself is slightly off-center. Once our design was finalized, the planner generated the list of cabinet boxes we needed.

Step 2: Assemble Your Ikea Cabinets

The first practical step was assembling the cabinet boxes. There are many helpful tricks and tips for installing Ikea kitchen cabinets; the key here is to build the boxes but hold off on attaching doors and drawer fronts until the cabinets are installed in place.

This is also a good time to double-check your measurements and make sure the assembled cabinets match the plan.

Step 3: Build Cabinet Bases

The standard Ikea legs would have left the cabinets too high for the window, so we built custom bases to achieve the exact height we wanted. We stacked 2×4 and 1×4 boards to build a low platform that matched the window height. These bases mainly serve as a solid surface for attaching the toe kick or baseboard later, though having them in place also helps protect the cabinet bottoms from wear and gives extra stability if kids clamber on them.

Step 4: Attach Cabinets To The Wall

Ikea cabinets use a metal rail system for wall mounting: the rail is attached to the studs, and the cabinet boxes hang from clips on that rail. Follow the manufacturer’s instructions for the rail height, and be sure to use a stud finder and a level to secure the rail firmly. Before hanging the rail we fastened a 2×4 to the studs at the back of the niche to give the 12″ deep cabinets more depth so they would sit properly in the 17″ deep space.

Hang each cabinet on the rail and verify alignment before locking them in. Small adjustments at this stage save a lot of finish work later.

Step 5: Add Filler Pieces

After hanging the cabinets (but before locking them together), add filler pieces between each box so the run looks like a single built-in unit. The niche and off-center window created some wider-than-normal gaps, so we cut filler strips from pre-primed wood to size on a table saw and clamped them in place. Secure the fillers to the cabinet sides with small screws, and use additional filler panels beside the window to conceal the metal rail system.

We purchased two extra factory filler pieces to cover the rail area and fitted them to match the surrounding cabinet faces.

Although this first layer of fillers conceals gaps, there’s usually one more layer required to achieve a finished, built-in appearance.

Step 6: Add More Filler Pieces

The initial fillers sit flush with the cabinet boxes but not with the cabinet doors. To get a custom, seamless look we added a second, thinner layer of filler that mimics the profile of the doors. Depending on the door style this can be a little tricky: our chosen doors have a subtle bevel, so we lightly sanded the filler edges to reproduce that detail before attaching them.

We glued the thin panels in place for a clean look, though you can also nail and fill them if you prefer. After finishing the joinery we painted all the cabinet faces and filler pieces in Soft Wool by Valspar to closely match the Ikea cabinet doors.

Step 7: Add Toekick or Baseboard

Because our cabinets sit low to accommodate the window, we installed a ripped-down baseboard under the cabinets, nailing it to the 2×4 base built earlier. If your cabinets are standard height you may not need to resize the baseboard, but this step ensures a polished transition from floor to cabinet.

Step 8: Add Counters

We considered butcher block, but decided a clean, all-white look would be best for this room. We bought wood project panels sized to cover each cabinet section, primed them, and painted them to match the cabinet finish. After the paint fully cured we secured the panels to the tops of the cabinet boxes from the inside with screws and allowed several days for the paint to fully harden before placing heavy items on them.

Step 9: Add Cabinet Hardware (Optional)

The final step was adding matching pulls to the doors and drawers. Carefully measure and mark hardware locations to ensure consistent placement across the run.

With the built-in cabinets complete, we finished the space by installing the complementary DIY floating shelves and styling the niche for a finished, usable feature wall.

For a full walkthrough of the room transformation, including before-and-after images, see our bonus room project documentation.

More Built-In Cabinet Projects

We’ve completed similar built-in projects in other areas of the house. If you’re seeking additional inspiration, consider variations such as customizing media cabinetry, using different door styles, or turning dressers into lower cabinet bases.

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• Turning dressers into bookshelf bases
• Built-in dresser using a Malm
• How to make Pax wardrobes look built-in