This guide explains how sash window restoration takes place in London’s heritage areas, what planning rules apply, how to distinguish between „Box Sash“ and „Spring Sash“ windows, and how Fenestrum supplies to homeowners, architects, contractors, and resellers restoring historic buildings.
Why are original wooden windows so important in London's conservation areas?
London's heritage areas exist to protect the architectural character of streets, squares and terraces where the collective buildings hold historical importance. Original timber sash windows are one of the most distinctive elements of this character. Their proportions, the profiles of the decorative glazing bars and the depth of the timber frame all contribute to the way light falls on Georgian or Victorian facades – an effect that cannot be replicated by uPVC or aluminium alternatives.
For most of London's conservation areas, replacing original windows with plastic presents both regulatory and financial risk. Conservation officers systematically reject applications for uPVC windows. Surveyors consistently note lower prices for heritage homes if their windows are not authentic. Furthermore, a well-made timber window – properly repaired or replicated to the original profile – typically lasts decades longer than plastic units, whilst retaining the carbon sequestration function of timber in the building rather than landfill.
Do you need permission to restore or replace windows in London's conservation areas?
In London's conservation areas, most external alterations that would normally be considered „permitted development“permitted development), are restricted by a local council issued „Article 4“ Direction. This means that changes visible from the street – including window replacements, changes to the pattern of boarding, or changes to materials or colours – will usually You need to obtain planning permission.
Identical. like-for-likeMaintenance and repairs typically do not require consent, provided that the same materials, profiles, and appearance are retained. Replacing a rotten windowsill or renewing a cord are repair works. However, changing a single-glazed window to a double-glazed unit, which alters the window profile, is considered an alteration and will likely require permission.
For Grade II listed buildings (Grade II listed buildingsSeparate permitListed Building Consentneeds to be obtained for almost any window replacement, including internal alterations. The local heritage officer is the first person to contact before any major works. Fenestrum can provide drawings, specifications and samples of finishes which can be submitted with applications for consent to carry out works.
A Box Sash window is a traditional type of sash window that incorporates a box-like structure. This structure typically houses the weights and pulleys that balance the sashes, allowing them to slide up and down smoothly.
„Box Sash“ is a traditional English sliding sash window, used throughout Georgian and Victorian London. The name refers to the hollow „box“ on each side of the window, which conceals the cast-iron weights that counterbalance the two sliding sashes. The sashes are connected to the weights by ropes running through brass pulleys at the top of the box. Fenestrum manufactures Sash Box windows with a standard 145mm and 155mm frame depth, tailored to the depth of the original building frames.
The terminology relevant to specifying this window is: parting bead separates the upper and lower sashes, Staff bead attach the bottom leaf to the frame, Meeting Rails There is a place where two leaves meet in the middle, horns are decorative bosses at the upper corners of the sash (a Victorian feature not usually found on early Georgian windows), and Glazing bars (glazing bars) divide the pane into smaller sections. „Fenestrum“ reconstructs each of these details according to historical drawings, joiner's templates or photographs of an existing window.
A spring sash window is a type of window that uses a spring mechanism to counterbalance the weight of the sash, allowing it to be opened and closed smoothly.
„The “Spring Sash" uses a spiral mechanism instead of weights and ropes. A compressed spring, hidden in a thin tube on each side, holds the sash in any open position. As no weight box is required, the frame can be significantly narrower. This was particularly suited to larger glass areas and the simpler profiles of the Edwardian and later periods.
These windows are chosen when the original windows already had springs, or when a heritage officer permits the use of a narrower frame. They are easier to operate, simpler to install during renovations, and create less stress on old masonry openings.
Repair or replace?
Conservation officers in London strongly favour repair over replacement. The approach generally follows four levels:
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Urgent prophylaxis This is useful when the wood is sound, but the window is in poor condition – draughty, sticking, with broken cords. Sashes are repaired, rebalanced, sealed, and repainted without altering their appearance.
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Partial renovation Used when decay is localised, most commonly in window sills or lower sections. The decayed section is cut out and replaced with appropriate timber using traditional joints.
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New door leaves in the existing frame A suitable solution when the frame (box) is in good condition, but the sashes are beyond repair. „Fenestrum“ manufactures new sashes to the precise dimensions and profiles of the original.
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Full replacement: This is an extreme measure in heritage zones, which usually requires permission. Once obtained, Fenestrum supplies full kits according to heritage specifications.
Production and timber specifications
The primary timber that „Fenestrum“ uses for London projects is glulam. Treated pine or fir (engineered redwood– Premium, slow-grown Northern European timber, laminated for dimensional stability. This is the most commonly used timber in the London heritage market due to its excellent paint-holding and durability properties. For projects requiring even greater durability, we manufacture from Sapele (and of birch) Accoya.
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Finishings The windows are painted at the factory using the Teknos system (any RAL colour or clear coatings).
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Glazing 24mm double glazing units, 18mm slimline units for heritage properties, or traditional single glazing (4mm or 6mm) with putty finish are available.
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Sanitization: Concealed brush seals are fitted into the architraves as standard – they are not visible from either the inside or the outside.
„Fenestrum“ Guarantees
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25 years – for medical treatments for fungus and insects.
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10 years – for manufacturing work.
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10 years – for white paint coatings.
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8 years – for double glazing.
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5 years – for dyeing/varnishing other colours.
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2 years – for damping inserts.
London boroughs we supply to
„Fenestrum“ supplies windows to heritage areas throughout London: Kensington, Chelsea, Hampstead, Highgate, Islington, Notting Hill, Camden, Westminster, Bloomsbury, Marylebone, Mayfair, Belgravia, Holland Park, Primrose Hill, Greenwich, Richmond, Temple, Kennington and Hackney.
Why choose a manufacturer from Lithuania?
Lithuania boasts some of the strongest wood processing traditions in Europe. Fenestrum's specialisation means the factory understands British profiles inside out – for us, it's not just a „window,“ it's a specific Georgian or Victorian-era variant with its own unique details. Our products are created exclusively for the UK market, adhering to British quality standards and local architectural styles.
Exports from Lithuania give homeowners and architects the opportunity to source factory-precision products at a price that smaller craftsmen often cannot match, without sacrificing quality.
Frequently asked questions about Fenestrum Timber Windows and Doors
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Do I need permission? For identical repairs – no. For replacement or changes to appearance (e.g., double glazing) – generally yes.
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What is the production lead time? Typically 8–12 weeks from order confirmation to delivery.
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Do you sell direct? Yes, we work with homeowners, architects, and contractors. We provide measurement and consultation services.
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How to get in touch? By phone, email, or video call. We can also arrange a visit to the factory in Lithuania.
Other steps
If you're restoring a historic building in London, get in touch. Fenestrum will prepare drawings and specifications for your heritage consultant and provide a detailed quotation.
About the company
„Fenestrum“ – a Lithuanian manufacturer of wooden windows with over 25 years of experience, producing top-quality „Sash“ and „Euro“ type windows for both the local market and the United Kingdom’s heritage areas, which have numerous specific requirements. The company specialises „Sash Box (with weights) and Sash Spring (with springs) In window manufacturing. All products are created in compliance with British architectural standards, precisely replicating the profiles, joinery, and finishes required in London's Conservation Areas.