Loft Conversion Costs: What Should Homeowners Budget?

Loft conversions can add a bedroom or office without extending the footprint — but costs vary significantly depending on roof type, dormer design, stairs and specification.

Published: June 2025 · Last reviewed: 24 June 2025

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The 10-second answer

How much does a loft conversion cost?

Typical conversions often range from roughly £30,000–£70,000+ depending on type and specification.

What is the cheapest type?

Velux (roof light) conversions within existing roof space are usually the most affordable.

Do dormers cost more?

Yes — dormers add construction cost and may add planning-related costs. See dormers and roof alterations.

Do I need to budget for planning?

Often not for simple conversions — but dormers and roof alterations may need planning permission.

Which route might apply?

You may be looking at permitted development if:

• Velux-only conversion within existing roof space.
• Small dormers within permitted development limits.
• Property has no special restrictions.

This does not automatically mean permitted development applies. Every property and proposal is different and still needs to be checked.

You may need planning permission if:

• Large dormers or mansard conversions.
• Listed buildings or conservation areas.
• Roof alterations exceeding permitted development.

Many projects require planning permission, but the answer depends on the individual circumstances.

For the planning side of things, see loft conversion planning permission and dormers and roof alterations. For building standards, see building regulations vs planning permission or building warrant vs planning permission.

Main cost areas

• Structural work — floor strengthening, steel beams
• Stairs — new staircase is often a significant cost
• Dormer construction (if applicable) — see dormers and roof alterations
• Windows and roof lights
• Insulation, plastering, electrics, plumbing
• Bathroom (if en-suite) — adds substantially
• Building regulations or building warrant fees
• Planning fees (if needed)

Conversion types and typical cost ranges

Indicative ranges (highly variable by region and specification):

• Velux conversion: often the lowest cost — uses existing roof shape
• Dormer conversion: mid-range — adds headroom and floor area; see loft conversion planning permission
• Hip-to-gable or mansard: usually highest cost — significant roof restructuring

These are broad guides only. Always obtain quotes for your specific property.

Planning-related costs

Many loft conversions do not need planning permission — but building regulations or a building warrant are almost always required. Budget for:

• Architect or technician drawings for building control
• Structural engineer calculations
• Planning application if dormers exceed permitted development — see loft conversion planning permission
• Listed building consent if applicable — see listed building consent explained

Typical cost examples

Cost ranges are indicative guides only — not quotes. Prices vary significantly by region, specification and installer.

Velux loft conversion with one bedroom

Might cost £30,000–£45,000+ depending on stairs complexity and region.

Rear dormer with en-suite

Might cost £50,000–£70,000+. Planning route depends on dormer size — see dormers and roof alterations.

Mansard conversion in London

Can exceed £80,000–£100,000. Planning permission usually required.

How to check your property

Before relying on general guidance, check the property and proposal together.

1. Confirm the property type. Houses, flats and maisonettes are often treated differently for permitted development.
2. Check whether the building is listed. Listed building consent and planning permission may both be needed.
3. Check whether the property is in a conservation area or other restricted area. Local restrictions such as Article 4 directions can remove some permitted development rights.
4. Review the size, position and design of the proposal. Measurements, previous additions and neighbour impact all matter.
5. Budget building standards approval. See building regulations vs planning permission or building warrant vs planning permission.

Plimbu helps bring these checks together so you can understand the likely planning route before going further.

Common mistakes

• Assuming a neighbour's project means yours is allowed — each property is assessed on its own facts.
• Ignoring previous additions that may have used permitted development allowances.
• Confusing planning permission with building regulations or a building warrant — see building regulations vs planning permission.
• Underestimating staircase cost and headroom requirements.
• Not budgeting for fire safety upgrades (doors, alarms).

Frequently asked questions

Is a loft conversion cheaper than an extension?

Often per square metre of new space — but depends on roof type and access.

Do loft conversions add value?

Usually yes, if done to good standard — but market varies.

How can I check planning before budgeting?

Use Plimbu with your address. See loft conversion planning permission.

Check your loft conversion planning route

Dormer design affects both planning and cost. Check your property before committing.

Check my property

This guide is general information, not formal planning advice. For property-specific checks, start with your postcode in Plimbu.