What Does an Extension or Loft Conversion Actually Cost in the UK?

Short answer: extension cost and loft conversion cost sit in similar ranges for smaller projects, but which one makes sense depends more on the property's layout and garden space than on price alone.

Homeowners weighing up the two often assume one is straightforwardly cheaper, when in reality the right choice usually comes down to what the property can physically support.

How Do Extension And Loft Conversion Costs Compare?

Both typically fall between £45,000 and £90,000 for a standard project, though the final figure depends heavily on size, specification, and site access.

A single storey rear extension usually costs £1,500 to £2,500 per square metre. A loft conversion generally costs £1,500 to £3,000 per square metre, with the higher end reflecting more complex roof alterations like dormers or mansards.

Neither option is consistently cheaper. The right comparison depends on what each square metre actually delivers for the specific property. A loft conversion that adds a much needed bedroom can outperform an extension of similar cost that only adds marginal kitchen space to an already adequate room.

Which Adds More Value, An Extension Or A Loft Conversion?

Both can add 15 to 25 percent to a property's value, though the exact uplift depends on execution quality and local buyer expectations.

An extension often adds more perceived value when it creates open plan kitchen and living space, since that's consistently one of the most sought after features for buyers.

A loft conversion tends to perform best when it adds a genuine extra bedroom, particularly in areas where family sized homes are in short supply relative to demand. In those markets, an additional bedroom can shift a property into a different buyer bracket entirely, adding value beyond the raw cost of the work itself.

What Determines Whether I Should Extend Or Convert My Loft?

A few practical factors usually settle the decision:

• Garden size, since a limited garden makes an extension less appealing regardless of cost.

• Existing roof height and pitch, which determines whether a loft conversion is structurally viable without major roofline changes.

• What space is actually needed, an extra bedroom typically points toward a loft, while more living space points toward an extension.

• Planning restrictions specific to the property, since Permitted Development eligibility can differ significantly between the two project types.

How Does Timeline Compare Between The Two Options?

A loft conversion generally takes 5 to 14 weeks for construction, extending to 12 to 25 weeks for larger or structurally complex projects.

A single storey extension typically takes 8 to 16 weeks, with double storey or more complex schemes running longer.

"Neither project timeline includes the planning and design phase beforehand, which can add several weeks depending on whether Permitted Development applies or a full application is required."

Factoring in that earlier phase matters when comparing timelines fairly, since a loft conversion with a straightforward Permitted Development route can start construction sooner than an extension needing full planning permission.

Can I Estimate My Loft Conversion Cost Accurately Before Committing?

Yes, through a free feasibility assessment that reviews the specific property's roof structure and planning status. For homeowners wanting to know how much is a loft conversion uk wide averages suggest, a detailed breakdown by conversion type gives a far more accurate starting point than a single national figure.

National averages tend to smooth over significant regional variation, particularly between London and other parts of the UK where labour and material costs differ considerably. A figure that looks accurate nationally can still be significantly off for a specific London postcode, which is why a property specific estimate matters more than a general benchmark.

Should I Do Both An Extension And A Loft Conversion?

Some homeowners combine both projects, either simultaneously or in phases. Doing both at once can be more cost efficient on shared costs like scaffolding and project management, though it requires a larger upfront budget.

Phasing the two, starting with whichever addresses the more urgent need, spreads cost over a longer period but means living through two separate construction phases instead of one. There's no universally right answer here. It comes down to budget flexibility and how much disruption a household is willing to tolerate at once versus spread over time.

The Bottom Line On Extension And Loft Conversion Costs

Cost alone rarely settles which project makes more sense. Garden space, roof structure, and what the household actually needs from the extra space matter more than a straightforward price comparison.

Getting both options properly costed against the specific property, rather than relying on generic averages, remains the most reliable way to choose between them with confidence.

A short feasibility conversation covering both options side by side tends to be more useful than researching each in isolation, since it surfaces trade-offs that only become clear when comparing them directly against the same property.