Why a window seat earns its keep

If you are lucky enough to have a bay window, you already own one of the most generous pieces of furniture in the house — it is simply wearing a plain wooden sill instead of a cushion. A proper seat pad turns that awkward ledge into the spot everyone gravitates towards: morning coffee, homework, a chapter before bed, or somewhere to sit and watch the rain. Shop-bought cushions rarely fit a bay exactly, so making your own means you get the depth, the thickness and the fabric you actually want, often for a fraction of the cost of a bespoke order.

Choose a firm foam pad

The single biggest mistake is choosing foam that is too soft. A window seat takes weight in one concentrated spot, so a squishy pad flattens within weeks and never springs back into shape. Look for upholstery foam with a density of roughly 25–30 kg/m³ — usually sold as firm or extra-firm seating foam.

• Thickness: 75mm for a seat used daily, 50mm if it is mostly decorative or only sat on now and then.
• Type: standard grey upholstery foam is perfectly good; high-resilience or "reflex" foam costs more but holds its shape for years longer.
• Cutting: many suppliers will cut to your measurements for a small fee. If you cut it yourself, an electric carving knife gives the neatest edge, though a bread knife works well with long, slow strokes.
• Comfort layer: wrap the foam in a layer of polyester wadding before it goes into the cover. It softens the edges, plumps the cushion and hides any wobble in your cutting.

Measure the bay carefully — then measure again

Bays are very rarely square, and a cushion that looks fine on paper can bulge or buckle once it is in place. Make a template from stiff card or overlapping sheets of newspaper, pressing it right into the corners of the recess.

• Record the widest point as well as the width at the back, and the depth at both ends if the bay is angled.
• Allow an overhang of around 20–25mm past the front edge, so the cushion sits proud and does not dig into the backs of your legs.
• Check the depth — 45–50cm is comfortable for sitting; anything shallower becomes a perch rather than a seat.
• Note window handles, radiator tops, skirting boards and the space your curtains need when stacked open. Clearance is easy to forget and impossible to adjust later.
• Leave a 10–15mm gap at the back against the wall. Air needs to move behind the cushion to reduce condensation on cold mornings.

Add roughly 1cm all round for wadding and fabric bulk, then trim the foam back if the finished cushion feels tight.

Sew a removable cover

A removable, washable cover is not a luxury — it is the difference between a cushion that lasts three years and one that lasts ten. Window seats collect sunlight, crumbs, pet hair and the occasional muddy footprint.

• Fabric: choose upholstery weight — cotton duck, linen blend, ticking, corduroy or a sturdy twill. Pre-wash and press it first so any shrinkage happens before you sew, not after.
• Shape: a box cushion (a top panel, a bottom panel and a continuous side strip) gives the crispest finish and holds the foam flat.
• Fastening: set a zip into the back panel or one side panel. Buy a zip a few centimetres shorter than the cushion and centre it, so there is no gaping at the corners.
• Simpler option: an envelope back with two overlapping flaps, secured with hook-and-loop tape or press studs. Less neat, but far quicker and just as washable.
• Piping: a piped seam around the top and bottom edges adds structure, helps the cushion keep its shape and disguises any slight unevenness in your stitching.
• Seam allowance: work with a consistent 1.5cm allowance and clip the curves before turning through.

Fitting and finishing touches

Fold the wadding around the foam, slide the whole parcel into the cover, then work the zip closed slowly, pushing the foam in with a flat hand rather than tugging at the fabric. Once it is in place, sit on it for a minute or two — foam needs to compress and settle before you judge the fit.

If the cushion slides about, stitch a few short lengths of wide ribbon to the back corners and tie them around the frame, or lay a strip of non-slip matting underneath. A couple of generous scatter cushions in a contrasting fabric finish the corner beautifully, and they are small enough to wash in a single load.

Washing, rotating and refreshing

Treat the cushion as you would any upholstery you want to keep looking sharp. Rotate it end to end every few weeks so the sun fades it evenly and the foam wears uniformly. Wash the cover at 30°C on a gentle cycle, reshape it while damp and line dry it out of direct sunlight — never tumble it, as heat shrinks both fabric and zip tape.

If you have the offcuts, run up a spare cover in a neutral linen and store it folded flat. When the first one is in the wash, the window seat stays usable, and you can ring the changes with the seasons for very little effort. That is the quiet luxury of making it yourself: a cosy, hardworking corner of the house, built to your measurements and easy to keep fresh.