What Is a Piggyback Loft Conversion? A Complete UK Guide
29, Dec, 2025
A piggyback loft conversion is becoming popular among homeowners who already have a loft conversion but need more space. Instead of expanding the whole roof, just add a little more space after the existing dormer without bearing the cost of disruption or a new dormer.
If you are in London or South West London, this blog helps to break down the cost and other permissions required.
What Is a Piggyback Loft Conversion?
It is a secondary loft extension that is built onto an existing dormer conversion. It is typically smaller than a full dormer but maximises the headroom or usable space for any area, like the en-suite bathroom, stairs, or storage. Piggyback conversions are often used where:
A full additional dormer is not possible
Planning restrictions limit roof alterations
Extra space is needed in only one part of the loft
Budget or structural constraints prevent a full extension
How Is a Piggyback Conversion Different from a Dormer?
The standard dormer loft conversion is usually extended across the rear roof slope, whereas the piggyback conversion is for a specific area. Key differences include:
Smaller footprint
Lower build cost
Less impact on the appearance
Often quicker to construct
Whilst it won’t provide as much space as a full dormer or L-shaped loft conversion, it can improve layout and comfort.
Piggyback Loft Conversion Cost in the UK 2026
The cost of this loft conversion in the UK depends on size, complexity, and location. In London, prices tend to be higher due to labour and access constraints.
Typical cost ranges:
Small piggyback extension: £15,000 – £25,000
Medium piggyback with internal alterations: £25,000 – £40,000
Piggyback with en-suite or structural upgrades: £40,000 – £55,000
These figures usually include:
Structural work and roofing
Insulation and weatherproofing
Windows or rooflights
Internal finishes (basic level)
Costs can increase if:
Steel beams are required
The existing dormer needs strengthening
The property is in a conservation area
Access is limited (common in terraced homes)
Do Piggyback Loft Conversions Need Planning Permission?
Yes, they can fall under Permitted Development, but it depends. The planning permission may be required if:
The extension exceeds permitted volume limits
The property is in a conservation area
The piggyback alters the roof shape significantly
The existing loft conversion already uses the full PD allowance
Even if planning permission is not needed, Building Regulations approval is always required. A Party Wall Agreement may also be necessary if the property is terraced or semi-detached.
Because rules vary by borough, especially across London, it is always best to seek help from a reliable UK building and construction company that knows local laws.
Roof Height and Structural Considerations
Many homeowners are curious to know whether their roof is suitable. Some of the general requirements include:
A minimum head height of around 2.2–2.3 metres
Strong existing dormer structure
Roof pitch that allows for extension
Adequate load-bearing capacity
Victorian and Edwardian homes may need additional structural reinforcement, such as steel beams, to support a piggyback conversion safely.
Design Ideas to Maximise a Piggyback Loft Conversion
To help you carefully plan this conversion, here are some popular design ideas:
Adding headroom for an en-suite bathroom
Improving stair access and safety
Creating built-in storage under eaves
Adding skylights to improve natural light
Extending a bedroom wall for a better layout
How Long Does a Piggyback Loft Conversion Take?
An average time is 4 to 6 weeks on site once approvals are in place. The timeline includes:
Survey and design: 2 - 4 weeks
Approvals and agreements: 4 - 8 weeks (if required)
Construction: 1 - 1.5 months
Does a Piggyback Loft Conversion Add Value?
Whilst it may not add as much value as a full dormer, it can:
Improve layout and usability
Make a loft bedroom compliant and comfortable
Increase appeal to buyers
Avoid the cost of moving
How Denham Crescent Can Help
Denham Crescent specialises in helping homeowners add extra space to their lofts where full home extension is not possible. Our team looks after the process from start to finish, so you don’t have to juggle multiple contractors for your work.
If you are considering a piggyback loft conversion, call us today, and let’s make it happen.
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