Including UK Building Regulations – a clear, honest cost guide for period homeowners
For many owners of period homes across London and the South East, sash windows are one of the most important and most misunderstood elements of their property.
They affect:
One of the most common assumptions we hear is that choosing between single glazed and double glazed timber sash windows is purely a personal preference.
In reality, UK Building Regulations play a decisive role.
In most cases, if you are replacing a timber sash window, you are legally required to install a double glazed sash window, unless your property is:
This article explains:
All using our transparent Perfect Pricing Page Framework, so you can make an informed decision with confidence.
Before discussing costs, it’s essential to understand the legal baseline.
Under Part L (Conservation of Fuel and Power) of the Building Regulations:
As a result:
If you are replacing a wooden sash window, it must be double glazed
You may install single glazed timber sash windows only if:
Outside of these scenarios, installing new single glazed sash windows is non-compliant, even if they visually match the originals.
This is why many homeowners are surprised to learn that double glazing isn’t an upgrade, it’s the default requirement.
These replicate original historic windows exactly:
They are usually specified for:
Because they don’t meet thermal standards, they’re often paired with:
These are purpose designed modern sash windows, not retrofits.
A properly made double glazed sash window includes:
When done properly, they offer:
Crucially, they are fully compliant with Building Regulations.
Double glazed sash windows must:
This alone increases cost compared to single glazing, which has no performance requirement in exempt buildings.
Cost impact:
Double-glazed sash windows typically cost 20–40% more per window than single-glazed equivalents.
Double glazing is not just “adding another pane”.
It requires:
Poorly designed systems result in:
High quality double glazed sash windows take significantly longer to manufacture.
For homes in Chelsea, Fulham, Putney, Richmond and similar areas, noise reduction is often as important as warmth.
Double glazing:
Achieving meaningful acoustic performance requires:
This adds further cost, but also daily quality of life benefits.
Even when double glazing is permitted, conservation officers often require:
Meeting these standards costs more than off the shelf systems, but avoids refusals and enforcement issues.
Lower end priced companies often:
This reduces cost but can be problematic in period homes where:
Cost savings often come from:
These windows may comply with Building Regulations, but won’t age gracefully.
Cheaper quotes frequently exclude:
This applies to both glazing types, but affects double-glazed units more severely if seals or balances fail.
Higher priced firms typically offer:
Premium firms factor in:
In high value homes, avoiding mistakes is often worth more than saving upfront.
General builders may:
The result can be:
Lower end double glazed sash windows often feature:
They technically meet regulations, but compromise the character of the property.
In realistic terms:
However, many homeowners find that:
Novells Sash Windows sit firmly in the premium but transparent category.
We are rarely the cheapest, and intentionally so.
Our pricing reflects:
We won’t:
Instead, we guide you through what’s permitted, what’s required, and what’s best for your home.
In most cases today, double glazing isn’t a choice, it’s the correct and compliant solution.
The biggest cost mistake homeowners make isn’t choosing double over single glazing; it’s misunderstanding what the law allows.
You should now clearly understand:
At Novells, our role is simple: to provide honest advice, heritage expertise, and transparent pricing, so you can move forward with confidence, knowing your windows are right for your home, today and for decades to come.
Under UK Building Regulations (Part L), when you replace a window, it must meet modern thermal performance standards, which single glazing does not. As a result, replacement sash windows are required to be double-glazed.
The only exceptions are:
Outside of these scenarios, installing new single-glazed sash windows would be non-compliant, even if they look identical to the originals.
Often, yes, but with conditions.
Many conservation areas allow double glazed timber sash windows provided:
Each local authority interprets policy differently, which is why experienced sash window specialists factor planning guidance and design compliance into their pricing. Cheaper systems frequently fail on detailing, leading to refusals or enforcement issues later.
The difference isn’t just the extra pane of glass; it’s the engineering.
Double-glazed sash windows require:
In real terms, high-quality double-glazed timber sash windows typically cost around 20–40% more than single-glazed equivalents, but they deliver superior comfort, energy efficiency, and long-term value.
Sometimes, but it depends on your priorities.
Secondary glazing can:
However, it also:
For non-listed homes, replacing windows with properly designed double-glazed sash windows is often the cleaner, more compliant, and longer-term solution.
As Co-founder of Novells Sash Windows, Philip has spent over 20 years perfecting the craft of designing, supplying, and fitting luxury wooden double-glazed box sash windows.
A family-run business led by Philip and his wife Nicola, Novells has fitted thousands of wooden windows across London and Surrey, with clients including Vivienne Westwood and the Victoria & Albert Museum.
