What Is the Difference Between a Loft Conversion and a Loft Extension?

If you are thinking about using your loft to gain more space, you will quickly encounter two terms that sound similar but describe very different approaches. Loft conversion. Loft extension.

They are often used interchangeably by homeowners and sometimes even by builders. But the distinction matters. It affects how much space you gain, how the rooms feel to live in, whether planning permission is required, and how much compromise is involved.

This article explains the difference clearly, using London homes as the context, so you can make a confident, well-informed decision.

The simple distinction

At its core, the difference is straightforward.

A loft conversion works within the existing roof shape.
A loft extension changes the roof shape to create new volume.

Everything else flows from that single decision.

Loft conversions. Working within the original roof

A loft conversion adapts the space already contained within the roof. The external roof line is largely retained, and the design focuses on making a limited volume usable.

This approach typically relies on:

• Rooflights to bring daylight into the space

• Careful insulation to improve comfort

• Joinery designed around sloping ceilings

• Accepting that head height and floor area are constrained

For many homeowners, this is what they imagine when they think of “doing the loft”.

Rooflight-led loft space
Retaining the pitched roof form while using large rooflights to improve daylight and perceived height.

In practice, even projects that look like simple conversions often involve more construction work than expected. Ridge heights may be adjusted, roof structures strengthened, or insulation strategies upgraded to meet modern standards. From a homeowner’s point of view, however, the experience remains the same. The space is shaped by the roof.

A loft conversion tends to work best when:

• The existing roof already has reasonable height

• The aim is one additional room rather than a full new storey

• Budget and disruption need to be tightly controlled

A well-designed loft conversion can feel calm and efficient. It succeeds by respecting its limits.

Why many London lofts cannot be converted at all

This is where many projects quietly change direction.

A large number of London houses, particularly terraces, have valley roofs. This is where roof slopes meet internally, creating awkward junctions and very limited central head height.

In these cases:

• There is little or no usable volume to convert

• Sloping geometry cuts across the space

• Even generous rooflights cannot solve the fundamental constraint

This is often the moment when homeowners realise they are not choosing between a conversion and an extension as a preference. They are choosing whether to create new roof volume or accept a compromised outcome.

Designing within roof constraints
Integrated joinery and careful detailing can improve usability, but the roof geometry still defines the space.

Loft extensions. Creating new roof volume

A loft extension removes the constraint by reshaping the roof.

Rather than adapting what is already there, the roof is altered to create usable space where none existed before.

This can include:

• A rear dormer or full-width dormer

• A mansard roof

• Raising or re-forming parts of the roof

• Rebuilding the roof structure to new proportions

The result is not just more space, but a different quality of space altogether.

Loft extension creating new volume
By reshaping the roof, the upper floor becomes a true additional storey rather than a constrained roof space.

Rooms formed through a loft extension tend to feel:

• More settled and balanced

• Easier to furnish and use day to day

• Better integrated with the rest of the house

This is also the point at which planning permission is usually required, as the roofline is being altered in a visible and material way.

For an overview of how planning applies nationally, see national planning guidance.

Why the difference is not always obvious from the street

From outside, the difference between a loft conversion and a loft extension is not always immediately apparent.

Many London homes have front parapet walls, meaning roof alterations sit behind them and are largely hidden from public view. Rear extensions are often not visible from the street at all.

Internally, however, the difference is immediately felt.

A loft conversion is shaped by the roof.
A loft extension reshapes the roof to suit the room.

That distinction affects comfort, daylight, layout, and how the space will feel to live in over time.

Which approach is right for your home?

The decision usually comes down to a few key questions.

• Is the existing roof volume genuinely usable?

• Are you comfortable designing around constraints, or do you want them removed?

• Is this a short-term gain or a long-term investment in how you live?

For some homes, a restrained, rooflight-led approach is enough. For others, particularly where valley roofs are involved, an extension is the only way to achieve a space that feels generous, calm, and future-proofed.

Understanding these choices early is part of how our full architectural service is structured.

A final thought

In reality, many London loft projects sit somewhere between a conversion and an extension. The language matters less than the outcome.

What matters is whether the roof geometry is working with you, or against you.

Understanding that difference early allows you to set realistic expectations, make better design decisions, and avoid costly compromises later on. That is also why it is worth understanding what architects’ fees actually cover before committing to a route.

If you would like early, impartial input on what is realistically possible in your own home, we offer early feasibility advice for your home.

That clarity is where good architecture begins.