Velux Loft Conversion Cost vs Dormer: Which Is Better Value?

If you are looking at converting your loft, these are the two most common options you will come across. Both add usable space. Both are popular in London. But they work differently, cost differently, and suit different homes.

Here is a clear comparison to help you decide which makes more sense for your situation.

What is a Velux loft conversion?

A Velux conversion, sometimes called a roof light conversion, is the most straightforward type. It keeps the existing roofline completely intact and simply adds windows flush into the roof slope.

Because you are not changing the roof structure, the build is faster, cheaper, and less disruptive. It is also more likely to fall under permitted development, meaning no planning permission is required in most cases.

The trade-off is headroom. You are working entirely within the existing roof space, so the usable floor area is limited to wherever the ceiling height is tall enough to stand or sit comfortably. In many London terraces and semis, that means a smaller usable room than people expect.

What is a dormer loft conversion?

A dormer extends outward from the existing roof slope, creating a vertical wall and a flat or pitched roof section. This dramatically increases both headroom and floor space compared to a Velux conversion.

Most dormers are built at the rear of the property. A rear dormer is the most common loft conversion in London because it maximises space, typically falls under permitted development, and does not affect the home's street appearance.

The build is more complex than a Velux conversion, which means it costs more and takes longer. But the space you gain is considerably greater.

If you want to understand the full range of conversion types before deciding, our loft conversion types guide covers everything from hip to gable and mansard options too.

The cost difference

This is where most people start, so here are realistic London figures for both.

A Velux loft conversion typically costs between £20,000 and £35,000 fully finished. The lower cost reflects the simpler build, less structural work, and shorter programme. You are essentially fitting windows, insulating, boarding out, adding a staircase, and finishing the space.

A dormer loft conversion typically costs between £35,000 and £60,000 fully finished. The additional cost covers the structural extension of the roof, new external walls, weatherproofing, and the extra internal volume that needs insulating, plastering, and finishing.

For a full breakdown of what drives these numbers, our loft conversion budgeting guide covers the hidden costs that sit on top of both types.

The value each one adds

Cost is only part of the equation. What matters is what you get for the money and what it does to your property value.

A Velux conversion adds value, but less than a dormer in most cases. Because the usable floor space is smaller, it is harder to fit a proper bedroom and ensuite, which is what buyers pay a premium for.

A dormer, particularly a rear dormer with a bedroom and bathroom, consistently adds more value in London. Agents and surveyors generally report that a well built dormer adds 15% to 25% to a property's value, which on most London homes means the conversion pays for itself and then some.

If the goal is maximising return on investment, the dormer wins for most London homes, even though it costs more upfront.

Which is better for your home?

The honest answer is that it depends on your roof.

A Velux conversion works well when the existing loft already has reasonable height throughout, typically a ridge height of at least 2.2 metres, and the floor area is large enough to create a comfortable room. It is also a good option when budget is the primary constraint or when planning restrictions make structural changes difficult.

A dormer makes more sense when headroom is limited, when you want to fit a bedroom and bathroom, or when you want to maximise the value added to the property. For most standard London terraces and semis, a rear dormer is simply the more practical and more valuable option.

The key question is whether your existing roof space has enough natural height. If it does, a Velux conversion can be excellent value. If it does not, you will be disappointed with the result and a dormer is worth the additional cost.

Our loft suitability guide helps you understand what your existing roof space can realistically support before you commit to either route.

Planning permission considerations

Both types usually fall under permitted development for rear extensions, but there are exceptions worth knowing about.

Velux conversions almost always qualify for permitted development because they do not alter the roofline. Rear dormers also typically qualify, provided they do not exceed certain size limits and you are not in a conservation area or a borough with additional restrictions.

Front dormers almost always require full planning permission, which adds cost and time. If you are in a conservation area, even rear dormers may need an application.

It is always worth checking with your local authority or speaking to a planning consultant before assuming permitted development applies to your specific property.

The straightforward verdict

If your budget is tight and your loft already has reasonable height, a Velux conversion offers solid value and a faster, cleaner build.

If you want to maximise space, create a proper bedroom and bathroom, and get the strongest return on your investment, a dormer costs more but delivers more. For most London homes, the extra spend is justified by the extra value it creates.

The best way to know for certain is to get a structural engineer to assess your roof space first. It costs a few hundred pounds and tells you clearly which route is actually viable for your home, before you spend time and money getting quotes for something that may not work.