Sash Window Replacement in London
When sash window repair is no longer viable — or when you want to significantly improve thermal performance, security or appearance — made-to-measure sash window replacement is the right solution. AMB Joinery manufactures and installs replacement sash windows across London, designed and built to match your property exactly. Every window is made to measure, every profile is matched to what exists, and every installation is FENSA-certified.
We are specialists in timber sash window replacement and do not fit uPVC or aluminium alternatives. If you want windows that look right in a London period property, last for decades, and increase the value of your home, timber is the only choice.
When Should You Replace Sash Windows?
We always assess whether repair is viable before recommending replacement. These are the situations where replacement genuinely makes sense:
- Extensive rot — more than 40 per cent of the structural timber has been lost; splice repairs are no longer sufficient
- Failed box casing — the cased frame that houses the counterbalance weights is structurally compromised
- Wrong profile — the existing windows are not original to the property (e.g. uPVC fitted by a previous owner) and you want to restore the correct appearance
- Significant energy upgrade — you want purpose-built double-glazed sash windows rather than a retrofit unit
- Whole-property programme — you are renovating and want new windows throughout with a consistent specification
For borderline cases, we are always honest. If repair is viable, we will tell you.
What Makes a Quality Sash Window Replacement?
Made-to-Measure Sizing
No two openings in a London period property are identical. Decades of settlement, repointing and previous window installations mean that even windows of nominally the same size will differ in width, height and reveal depth. AMB Joinery measures every opening at three points on each axis before manufacturing begins. Our tolerances are ±1mm.
Off-the-shelf sash windows — even those marketed as "period style" — require packers, reveals and infills to make them fit. The results are never as good as a window made to the opening.
No two openings in a London period property are identical. Decades of settlement, repointing and previous window installations mean that even windows of nominally the same size will differ in width, height and reveal depth. AMB Joinery measures every opening at three points on each axis before manufacturing begins. Our tolerances are ±1mm.
Off-the-shelf sash windows — even those marketed as "period style" — require packers, reveals and infills to make them fit. The results are never as good as a window made to the opening.
Matching Original Profiles
The character of a sash window is defined by the moulding profiles: the ovolo or ogee on the sash stiles and rails, the glazing bar profile, the size and position of the horns, the width of the meeting rail. AMB Joinery can replicate any profile from a physical sample or a detailed drawing. We carry tooling for the most common London period profiles and can manufacture bespoke cutters for unusual details.
If one window in a row needs replacing, we match it exactly to the others. If all windows are being replaced, we work from the surviving details of the existing windows or from historic photographs.
The character of a sash window is defined by the moulding profiles: the ovolo or ogee on the sash stiles and rails, the glazing bar profile, the size and position of the horns, the width of the meeting rail. AMB Joinery can replicate any profile from a physical sample or a detailed drawing. We carry tooling for the most common London period profiles and can manufacture bespoke cutters for unusual details.
If one window in a row needs replacing, we match it exactly to the others. If all windows are being replaced, we work from the surviving details of the existing windows or from historic photographs.
Timber Species Selection
We manufacture replacement sash windows in three main timber species:
Accoya — Our recommended specification for most London replacement projects. Accoya is radiata pine that has been acetylated — a chemical process that modifies the wood's cell structure, making it dimensionally stable, highly durable and resistant to rot and insects. It carries a 50-year above-ground durability guarantee. Accoya is Class 1 for durability, accepts paint exceptionally well, and is the preferred species for exposed locations and ground-floor windows.
European Redwood — A traditional and cost-effective choice. Properly specified (select grade, kiln dried) and well maintained, European redwood sash windows have a long service life. It is the correct specification for budget-conscious projects where Accoya's premium is not justified.
Hardwoods — Oak, sapele and iroko are available for specific requirements, including external doors and staircases where hardwood is preferred.
We manufacture replacement sash windows in three main timber species:
Accoya — Our recommended specification for most London replacement projects. Accoya is radiata pine that has been acetylated — a chemical process that modifies the wood's cell structure, making it dimensionally stable, highly durable and resistant to rot and insects. It carries a 50-year above-ground durability guarantee. Accoya is Class 1 for durability, accepts paint exceptionally well, and is the preferred species for exposed locations and ground-floor windows.
European Redwood — A traditional and cost-effective choice. Properly specified (select grade, kiln dried) and well maintained, European redwood sash windows have a long service life. It is the correct specification for budget-conscious projects where Accoya's premium is not justified.
Hardwoods — Oak, sapele and iroko are available for specific requirements, including external doors and staircases where hardwood is preferred.
Glazing Options
All our replacement sash windows can be glazed to specification at the point of manufacture:
Single pane (4mm clear or obscure) — for listed buildings, conservation areas requiring like-for-like, or where the planning authority specifically requires single glazing.
Slim double-glazed units — 12mm total sealed units (4mm glass / 4mm cavity / 4mm glass) that maintain the traditional profile; available in standard, acoustic and low-e specifications
Fineo vacuum glazing — 6.7mm total thickness; excellent thermal performance; suitable for the most restrictive conservation area and listed building requirements
See our double glazing for sash windows page for full details on glazing options.
All our replacement sash windows can be glazed to specification at the point of manufacture:
Single pane (4mm clear or obscure) — for listed buildings, conservation areas requiring like-for-like, or where the planning authority specifically requires single glazing.
Slim double-glazed units — 12mm total sealed units (4mm glass / 4mm cavity / 4mm glass) that maintain the traditional profile; available in standard, acoustic and low-e specifications
Fineo vacuum glazing — 6.7mm total thickness; excellent thermal performance; suitable for the most restrictive conservation area and listed building requirements
See our double glazing for sash windows page for full details on glazing options.
The Replacement Process
Replacing sash windows with AMB Joinery follows six stages:
Free site survey
We measure every opening, assess existing conditions, discuss glazing specification and profile requirements, and photograph the existing windows
Design and specification approval
We provide detailed drawings and a written specification for your approval before any manufacturing begins
Manufacturing
All windows are made to order in our workshop; lead time is typically 6-10 weeks from specification approval
Existing window removal
Existing windows and frames are carefully removed; any minor making-good to reveals and linings is completed
Installation
New windows are fitted, balanced, adjusted and tested; ironmongery fitted; any external making-good completed
FENSA certification
All replacement window work is registered with FENSA; you receive your certificate within 30 days of completion
Replacing uPVC with Timber Sash Windows
A significant proportion of London period homes had their original timber sash windows replaced with uPVC units in the 1980s and 1990s. This is now widely considered to have been a mistake — aesthetically, technically and in terms of property value.
uPVC cannot replicate the slim sightlines, the moulding profiles or the traditional proportions of a timber sash window. It changes colour over time, cannot be repainted, and typically lasts 20-25 years before requiring replacement again. In many conservation areas, uPVC sash windows should not have been installed in the first place and may not receive planning permission for like-for-like replacement.
Switching back to timber is increasingly common among London homeowners who want to restore the correct appearance of their property. AMB Joinery has extensive experience of this work, including managing the interface with planning officers where the uPVC installation was originally non-compliant.
FENSA Compliance
All sash window replacement work carried out by AMB Joinery is registered with FENSA. This means the work self-certifies compliance with building regulations (principally thermal performance under Part L), and you receive a FENSA certificate that demonstrates compliance to solicitors and mortgage lenders when you sell your home. Find out more on our FENSA compliance page.
Frequently Asked Questions
How much does sash window replacement cost in London?
Replacement sash windows are a significant investment. The cost depends on the number of windows, timber species, glazing specification and access requirements. We provide detailed written quotes following a free site survey. As a guide, Accoya box sash windows with slim double glazing are more expensive than standard redwood with single glazing, but carry a 50-year durability guarantee.
Replacement sash windows are a significant investment. The cost depends on the number of windows, timber species, glazing specification and access requirements. We provide detailed written quotes following a free site survey. As a guide, Accoya box sash windows with slim double glazing are more expensive than standard redwood with single glazing, but carry a 50-year durability guarantee.
How long does it take to replace sash windows?
Manufacturing lead time is typically 6-10 weeks. Installation of a typical Victorian terrace (6-8 windows) takes 2-3 days on site.
Manufacturing lead time is typically 6-10 weeks. Installation of a typical Victorian terrace (6-8 windows) takes 2-3 days on site.
Can I replace my sash windows in a conservation area?
Usually yes, but the requirements depend on whether your street has an Article 4 Direction and whether your property is listed. We will establish the planning position as part of the survey.
Usually yes, but the requirements depend on whether your street has an Article 4 Direction and whether your property is listed. We will establish the planning position as part of the survey.
Should I replace with timber or uPVC?
For period properties in London, timber. It is the only material that correctly replicates the character of original sash windows, is accepted by planning authorities, and increases rather than reduces property value. AMB Joinery does not install uPVC windows.
For period properties in London, timber. It is the only material that correctly replicates the character of original sash windows, is accepted by planning authorities, and increases rather than reduces property value. AMB Joinery does not install uPVC windows.
Do I need planning permission to replace sash windows?
In most cases, no — replacement is permitted development. Exceptions are listed buildings (always require consent) and properties in conservation areas with Article 4 Directions (require planning application). We will advise you on your specific situation.
In most cases, no — replacement is permitted development. Exceptions are listed buildings (always require consent) and properties in conservation areas with Article 4 Directions (require planning application). We will advise you on your specific situation.
Get a Free Survey
Request a free survey and we will visit your property, assess your windows, discuss the specification and provide a detailed written quote. We work across all London boroughs and typically respond to enquiries within 24 hours.
