Understanding Loft Conversion Costs in London (2025 Update)

Updated for 2025

If you’re planning a loft conversion, one of the first questions is always the same:
how much is this actually going to cost?

As an architectural practice working on lofts across London and the South East, we see real build costs week in, week out – from small terraces to large detached homes with complex roofs.
This updated guide is designed to help you understand loft conversion costs in practical terms, based on typical projects we see rather than theoretical price-per-square-metre figures.

All figures below are approximate build costs only and will vary depending on specification, contractor, access and market conditions – but they’ll give you a solid, realistic starting point.

Typical loft conversion costs in London (2025)

Below is an at-a-glance overview of typical loft conversion costs we see for different property types in London.

1. Terraced houses

Small terrace – loft conversion without a rear dormer

• Typical use: 1 bedroom, sometimes with an en-suite (if space allows)
• Typical build cost: around £60,000

Small terrace – loft conversion with a rear dormer

• Typical use: 1 good-sized bedroom with en-suite
• Typical build cost: around £70,000–£80,000

2. Semi-detached houses

Hip-to-gable with rear dormer – semi-detached

• Typical use: 1 large bedroom with en-suite, sometimes with extra storage space
• Typical build cost: around £80,000–£90,000

Larger semi-detached lofts

• More complex layouts or additional space, such as a bigger bedroom, larger en-suite, or dressing area
• Typical build cost: around £100,000

3. Detached houses

Detached properties with complex roof structures

• Multiple roof pitches, valleys, or more extensive dormer work
• Often used to create a generous master suite or occasionally two bedrooms
• Typical build cost: around £100,000–£150,000

4. Loft conversions to flats

Flats are a different story to houses:

• Almost always require full planning permission
• Need freeholder’s consent (licence to alter, etc.)
• You must check who actually owns the loft space in the lease – it may be retained by the freeholder

Build costs can be similar to houses, but the legal and planning side is usually more involved, and access/logistics can also affect price.

What do you actually get for that money?

In most London homes, a loft conversion is best used to create:

• One generous bedroom with en-suite, often as a master suite
• In larger roof spaces (especially semis and detached houses), sometimes two bedrooms, or a bedroom plus office/guest room

Typical configurations we see:

• Terrace: one double bedroom + shower room
• Semi-detached: one large bedroom with en-suite and storage, or bedroom + study
• Detached: one large master suite or two smaller bedrooms with a shared or separate bathroom, depending on roof size and layout

Good design at the early stage makes a huge difference: where the stair arrives, how much head height you have, and how much of the new floor is truly usable, not just low, awkward corners.

Loft height: the critical feasibility check

Before you fall in love with Pinterest images, loft height is the first thing to check.
Understanding whether your existing loft space is tall enough can save you a lot of time and disappointment.

A useful rule of thumb:

• Ideally you want more than 2.4m from the existing loft floor/ceiling joists up to the underside of the ridge line before works.
• The finished stair will need at least 2.0m head height over it to meet Building Regulations.

If the height is borderline, there are options (lowering ceilings below, raising the ridge in some cases), but they come with extra cost and potentially planning implications.

If you’re unsure about your loft height, we can help measure it properly during a feasibility visit and advise whether the project is realistic and what compromises might be needed.

Permitted Development vs planning permission for loft conversions

For many houses in London, Permitted Development (PD) rights are available for loft conversions, which can simplify the process and sometimes speed it up.

In simple terms:

• Most of the time, Permitted Development is suitable for typical rear dormer loft conversions on houses.
• If the volume of the new loft space exceeds around 50 cubic metres (or 40 cubic metres for some properties), you are likely into full planning permission territory but these applications may be higher risk.
• If your property is in a conservation area, you often cannot use PD for roof alterations and will need planning permission for the loft conversion.

Even when PD is available, we usually recommend applying for a
Lawful Development Certificate
so you have official confirmation from the council that the work is lawful – helpful for peace of mind and future sale.

If you live in a flat, PD for roof extensions generally won’t apply, so you should assume you’ll need planning permission,
freeholder’s consent, and a careful review of your lease to confirm ownership of the loft space.

Other costs to factor in (beyond the build price)

The build figures above are only part of the story. To properly understand loft conversion costs, you also need to allow for:

• Professional fees
  Architect / designer, structural engineer, party wall surveyor, planning and Building Control fees.
• Regulation-driven upgrades
  Fire doors to the stair route, linked smoke alarms, extra insulation requirements.
• Finishes & fixtures
  Flooring, tiling, sanitaryware, bespoke wardrobes/joinery, lighting and electrical upgrades.

We typically recommend a contingency of 10–15% on top of the estimated build cost to cover unknowns and changes – particularly in older houses where surprises behind the plaster are common.

For a more detailed breakdown of everything to think about, see our
full loft conversion checklist.

How we approach loft conversions at Detailed Planning

Our typical process for loft projects in London looks like this:

1. Feasibility & early advice
   Measure loft height, check basic structure, and carry out a quick review of planning, Permitted Development and conservation constraints for your address.
2. Concept design
   Explore layouts (one vs two bedrooms, en-suite options), and check stair position and head height properly.
3. Planning / Lawful Development Certificate
   Decide whether PD is available or full planning is required, prepare drawings and supporting documents, and act as your agent throughout the application.
4. Technical design & Building Regulations
   Prepare detailed drawings for Building Control and coordinate with the structural engineer and other consultants.

We regularly work with local builders and design in a way that’s efficient to build, which helps keep costs under control.

You can see examples of similar projects on our
recent loft conversion projects
and read what our clients say about us on our
Google reviews page.

Is a loft conversion worth it?

In many London homes, a loft conversion is one of the best value ways to add space and value, especially where:

• You already have good garden space and don’t want to lose more with a rear extension.
• You’re looking for a master suite retreat away from the kids’ bedrooms.
• You want to future-proof the house for resale.

The key is going into it with realistic cost expectations, proper planning advice, and a design that makes the most of the space you’re paying to build.

Ready to explore your loft conversion?

If you’re considering a loft conversion in London and want project-specific guidance on cost, planning and feasibility, we can:

• Check your loft height and roof form.
• Advise whether Permitted Development or planning permission is more appropriate.
• Give a realistic cost band for your type of property (terraced, semi-detached, detached or flat).
• Outline the next steps from first sketches through to technical drawings.

Book a call with us
or explore our
recent loft conversion projects
to see what’s possible in homes similar to yours.