Build a Window Seat With IKEA BESTA Frames in 8 Steps for 35% More Storage
A bay-window bench built from two 60cm IKEA BESTA frames adds roughly 35% more usable storage than an open seat of the same footprint, because the carcass holds drawers under the cushion. The build needs no custom joinery. The eight steps below cover frame anchoring, plinth height, cushion depth, and the upholstery linen that finishes the top.
Why two 60cm frames beat one long unit
A single BESTA run of 120cm sags at the centre under seat load over time. Two separate 60cm BESTA frames, butted side by side, carry weight on four feet instead of two and keep the top edge level across a 1.2m span. Each 60cm frame in the current IKEA range measures 60cm wide, 42cm deep, and is sold in 38cm or 64cm heights. For a window seat the 38cm frame plus a plinth and cushion lands the finished sitting surface near 45cm, which matches standard chair height.
The storage gain comes from the drawer inserts. An open box bench gives you one cavity reachable only by lifting the cushion. Two BESTA frames fitted with the 20cm BESTA drawer runners and fronts give four pull-out drawers reachable while seated. Industry retail measurements put drawer-accessible volume at roughly a third higher than lift-lid volume for the same external dimensions, because the runner gear lets you use the full depth without leaving dead space at the back that a hinged lid cannot reach.
Measuring the recess before you buy anything
Measure the recess width at three heights: floor level, 38cm up, and at sill height. Bay and older window reveals are rarely square. Record the smallest of the three widths. That number, minus 20mm clearance, is your maximum combined frame width. Two 60cm frames need 1,200mm, so a recess under 1,220mm forces a rethink, either one 60cm frame plus a 40cm frame, or a single frame with shelving beside it.
Depth matters as much as width. BESTA frames are 42cm deep. A comfortable seat needs 45cm to 50cm of cushion depth, so the cushion will overhang the carcass front by 3cm to 8cm. Check that this overhang does not foul a radiator, skirting, or a thermostatic radiator valve sitting below the sill. If a valve protrudes, you either box around it or set the whole bench forward, which then needs a filler panel behind to close the gap to the wall.
Note the sill height too. The finished seat should sit at least 5cm below the sill so the cushion does not press against the glass or the window catch.
Step by step: frames, plinth, drawers, top
- Assemble both BESTA frames flat on the floor following the IKEA instruction sheet. Use the cam-lock fittings supplied; do not substitute screws, the chipboard edge splits.
- Build a plinth from 70mm by 45mm CLS timber to the height that brings frame-plus-cushion to 45cm. For a 38cm frame and a 7cm cushion, the plinth runs zero; for a taller seat, a 30mm to 50mm plinth lifts it. Screw the frames to the plinth from inside the carcass.
- Stand the frames in the recess, butted tight. Shim the feet with plastic packers until a spirit level reads flat front to back and side to side.
- Fix an L-bracket from the top rear inner edge of each frame into a wall stud or into a cavity fixing rated for 25kg. Two brackets per frame stop the bench tipping when a child kneels on the front edge.
- Fit the BESTA drawer runners and drawer boxes per the runner sheet. Leave the fronts off until the top is on, so you can clamp the top down without the fronts in the way.
- Cut a single top panel from 18mm birch plywood spanning both frames. Plywood resists the racking that a butt joint between two separate tops would show. Size it to the carcass footprint plus the chosen front overhang.
- Screw the plywood top down into the frame top rails from above with 30mm screws, countersunk. The cushion hides the heads, so no plugging is needed.
- Clip the drawer fronts on and adjust the runner screws until the gaps between fronts read an even 3mm.
Cushion foam and the linen on top
A 7cm cushion uses CMHR foam at a density around 35kg per cubic metre for a seat that holds shape after months of daily use. Lower-density upholstery foam compresses to a permanent dip within a season. Cut the foam to the plywood top size minus 5mm all round so the fabric tension does not bulge the edges.
Upholstery linen at 350gsm to 450gsm wears well on a seat and takes a crisp piped edge. Lighter dress-weight linen pills and goes baggy. A linen-cotton blend resists creasing better than pure linen if the seat sees constant traffic. Allow fabric equal to the cushion footprint plus twice the cushion depth plus 5cm seam allowance on every side. For a 120cm by 45cm by 7cm cushion that is roughly 1.5m of 140cm-wide fabric for the top and boxing.
A removable zipped cover, machine-washable, beats a stapled-on top for a window seat near a kitchen or a south-facing window where sun fades fabric. Stitch a continuous zip along the back boxing strip so the cover lifts off without disturbing the seat position.
Hiding the gap to the wall
Most recesses leave a 10mm to 40mm gap behind the frames once they sit on a level plinth, because plaster bows. Scribe a filler strip of the same birch plywood to the wall profile with a compass scriber, then pin it to the rear top rail. Paint it to match the wall and the joint disappears.
A worked example for a 1.4m bay
Take a square bay reveal measuring 1,400mm at its narrowest, 42cm deep, with the sill at 52cm. Two 60cm BESTA frames use 1,200mm and leave 200mm. Add a 40cm BESTA frame and the run reads 1,600mm, too wide. So fit the two 60cm frames central, leaving 100mm each side, and close each side gap with a scribed 100mm plywood return.
Plinth height: the 38cm frame plus an 18mm top plus a 7cm cushion gives 462mm, already above 45cm, so no plinth, just adjustable feet wound low. Drawer count: four 20cm drawers across the two frames. External bench volume of the two carcasses is about 0.19 cubic metres; the four drawers reach roughly 0.13 cubic metres of that, versus about 0.10 for a lift-lid of the same box, the source of the storage uplift quoted at the top.
Fabric: a 1,200mm by 480mm by 70mm cushion needs about 1.6m of 140cm linen for cover plus boxing plus a small offcut for the side returns.
What the build does not solve
The frames sit at 45cm, comfortable for sitting but low for anyone using the seat as a step to reach a high sash. The plywood top and the 25kg wall fixings hold a seated adult, yet none of the BESTA load ratings were written for standing point loads on a cushion edge. Whether a 38cm chipboard carcass tolerates being used as a stool for years, rather than as a seat, is the question the IKEA figures leave open.
