Start with what you already have

Before you ring a joiner, spend a damp Saturday morning looking properly at the windows you have. Old cottage windows are rarely a matched set. One may be an original sliding sash, another a twentieth-century casement, and a third a well-meaning replacement from the 1980s with the proportions of a bus shelter. Knowing what you are dealing with changes everything about what you should order. Take a notebook and record the number of panes in each opening, the width of the glazing bars, the depth of the reveal from the outer face of the wall to the inner face of the frame, and the position of the frame within that reveal. Old frames usually sit well back, leaving a deep internal sill and a shadow line outside. Push a new frame flush to the outer brickwork and the cottage loses that hollow-eyed, thick-walled look that makes it feel old.

• Count panes and note which bars are vertical and which are horizontal.
• Measure bars with a ruler, not by eye — 20mm and 35mm read very differently in daylight.
• Photograph each window from inside and out, straight on and at an angle.
• Note the sill: stone, brick, tile, or timber, and how the frame sits on it.

If your windows are listed, or you are in a conservation area or an Article 4 direction area, speak to your local conservation officer before you order anything. Most are far more interested in keeping proportions, materials and details than in preventing warmth, and a sensible conversation early will save a painful one later.

Timber frames and what to specify

Timber remains the right answer for almost every old cottage, and not for nostalgic reasons. It is repairable, it can be painted any colour, it moves with the building rather than against it, and it can be made to any dimension your odd walls demand. Softwood is perfectly adequate if it is properly seasoned and painted, though for south-facing or exposed elevations, a durable hardwood or an acetylated or heat-treated timber will hold its shape better and need less attention.

Ask for frames that match the original sightlines. That means a frame section no chunkier than it needs to be for the glazing you want, mouldings copied from an existing member, and joints that are pegged or mortised rather than stapled. Ask for the frame to be set at the same depth in the reveal as the old one, and for a drip or throat on the underside of the sill so water is thrown clear of the wall.

Glazing bars and proportions

The single biggest giveaway of a modern window is fat glazing bars. A bar of 60mm looks clumsy beside an original of 22mm, especially in a small cottage window where the glass is barely 300mm wide. Slim bars are available and cost little extra.

If you want double glazing, a slim unit of 4mm glass, a 6mm cavity and 4mm glass, total around 14mm to 16mm, will sit comfortably in a bar of 24mm to 28mm. In many cottages a slightly wider bar reads perfectly well, because the eye compares the whole opening rather than the millimetre. Never let a joiner persuade you to reduce the number of panes to "make it work". Two panes side by side where four once sat will look wrong from the lane and wrong from the sofa.

Where glass area matters, ask for the largest possible opening within the frame. Stepped rebates and putty glazing rather than beading will each give you a few extra millimetres of light, which matters in a low-ceilinged room.

Hardware and the details that sell it

Ironmongery does more for authenticity than almost anything else, and it is cheap to get right. Choose solid brass or blacksmith-made iron rather than plated alloy, and match the finish already in the house. Sash windows with weights are lovely but require space in the box frame; spiral or spring balances are quieter and easier to fit, though they feel different under the hand. If you are keeping sashes, retain the staff and parting beads and fit discreet brush seals — a well-sealed original sash performs far better than a leaky modern copy.

• Match handles, stays and catches across the elevation, but use period-appropriate patterns for older openings.
• Fit trickle vents discreetly, high in the frame, or use a background ventilator elsewhere — cottages need air.
• Keep external sills and heads simple; deep ornate hoods rarely suit a small cottage.
• Paint in a breathable, microporous finish such as a linseed or alkyd system, and allow timber to dry between coats.

Making them warm without sealing them shut

Replacement windows alone will not make a stone cottage warm. The gain comes from the whole assembly. Insulate the reveal, fill the gap between frame and wall with a flexible, breathable sealant or mineral wool, and make sure internal plaster gives the frame somewhere to sit. Curtains on a proper pole with a generous overlap, or lined shutters closed at dusk, will do as much as an extra millimetre of glass. For listed windows, secondary glazing is often the sensible compromise, and it is far less invasive than removing historical joinery.

On performance, replacement windows in England are generally expected to achieve a U-value of around 1.4 W/m²K, though conservation areas and listed buildings are frequently granted some flexibility where a like-for-like timber window cannot reach it. Take the advice of your joiner and your conservation officer, and do not sacrifice a hundred years of character for a number on a certificate.

Repair first, replace second

Many cottage windows need less than owners assume. Rot is often localised to the bottom rail or the sill, both replaceable. Sash cords and weights can be renewed. Draughts can be chased out in a morning with new beads and seals. A good joiner will repair a serviceable window for a fraction of the cost of a new one, and the repaired original will almost always look better. When replacement really is the answer, take the measurements, insist on slim sections, and keep the proportions. Done properly, nobody will notice the work — only that the room is quieter, the draughts have gone, and the cottage still looks like itself.