Loft conversion cost in London and Essex (2026)
A loft conversion in London and Essex costs between £40,000 and £95,000 in 2026. Where you land inside that range is not random. It comes down to the type of conversion your roof will take, the floor area you gain, whether you add a bathroom, and the level of finish you choose.
A Velux conversion sits at the bottom of the range because it changes nothing structural. A full mansard sits at the top because it rebuilds the roof. Everything else falls somewhere between those two.
This guide sets out the real 2026 cost of each conversion type, what pushes a quote up, and what should already be in your budget before you invite anyone to price the job. We build loft conversions across North London, Brentwood, Chingford and the wider Essex area, so these figures reflect London and South East pricing. National averages run roughly 25 to 30 percent lower and are not much use to you here.
Loft conversion cost by type
| Conversion type | Best suited to | Typical London and Essex cost | Build time |
|---|---|---|---|
| Velux (rooflight) | Lofts with good existing head height | £40,000 to £50,000 | 4 to 6 weeks |
| Rear dormer | Terraced and semi-detached homes needing headroom | £50,000 to £65,000 | 6 to 8 weeks |
| Hip-to-gable | Semi-detached and detached homes with a sloping side roof | £60,000 to £75,000 | 8 to 12 weeks |
| L-shaped dormer | Victorian and Edwardian terraces with a rear outrigger | £65,000 to £85,000 | 8 to 10 weeks |
| Mansard | Period terraces wanting a full extra storey | £70,000 to £95,000 and above | 10 to 16 weeks |
Ranges include labour and materials for the structural build, staircase, insulation, glazing, plastering and standard finishes. They exclude VAT, high-end fittings and any en-suite.
What decides which type your roof will take
Most homeowners start by choosing a conversion type. That is the wrong way round. Your roof decides, and the cost follows.
Measure the height from the ceiling joist to the ridge at the highest point. If you have 2.4 metres or more, a Velux conversion will usually work and you have the cheapest option available. Between 2.2 and 2.4 metres you are into dormer territory, because you need to raise the roof to create standing height. Below 2.2 metres, a Velux is off the table and you are looking at a dormer, a hip-to-gable, or a mansard.
Roof shape matters just as much. A hipped roof, where the side slopes inward, wastes a lot of usable space. Squaring it off with a hip-to-gable recovers that volume and is why the figure sits higher. A Victorian terrace with a rear outrigger suits an L-shaped dormer, which wraps the new space around the back of the house and typically adds a double bedroom plus an en-suite rather than a single room.
What changes the price
The type of conversion. More alteration to the roof means more cost. A Velux changes nothing structural. A dormer extends the slope. A hip-to-gable extends sideways. A mansard rebuilds the roof almost entirely.
Your roof structure. A traditional cut roof, common in homes built before the 1960s, has open rafters and is straightforward to convert. A modern trussed roof has W-shaped webbing filling the loft void, and every one of those trusses is holding the roof up. Removing them means new steel beams and structural support, which adds several thousand pounds.
Adding a bathroom or en-suite. Plumbing, drainage and waterproofing typically add £5,000 to £10,000. Where the new bathroom sits relative to your existing soil stack makes a real difference here.
Staircase position. A staircase that drops neatly over the existing flight is the cheap version. One that has to be relocated, or squeezed into a landing that then needs reworking, costs more and eats floor area on the storey below.
Finish level. Standard rooflights and off-the-shelf joinery keep you near the bottom of each range. Bespoke glazing, a feature staircase and fitted cabinetry move you towards the top quickly.
Professional and legal fees. A structural engineer in London runs £1,500 to £4,000. A party wall surveyor, needed for terraced and semi-detached homes, runs £800 to £1,500 per neighbour.
A worked example
A three bedroom terraced house in Chingford, rear dormer conversion, one double bedroom with an en-suite.
The build itself comes in at £58,000, which sits in the middle of the dormer range. Architectural drawings add £1,800. Structural calculations add £1,200. Building Regulations sign-off adds £750. One party wall surveyor covering both attached neighbours adds £1,400. The en-suite adds £7,000. VAT at 20 percent applies to the build and most of the fees.
The total lands close to £84,000. The headline dormer figure was £58,000. That gap is the single most common reason a loft conversion budget goes wrong, and it is entirely avoidable if you price the whole job rather than the build alone.
What a quote should cover, and what sits outside it
A design and build quote normally covers the structural work and steels, the staircase, insulation, the roof windows or dormer construction, plastering, electrics, standard flooring and decoration.
Budget separately for VAT at 20 percent, architectural drawings at £1,000 to £3,000, structural calculations, Building Regulations at £500 to £1,000, any party wall surveyor, an en-suite, and any high-end finishes you have set your heart on.
If a quote does not tell you which side of that line each item falls, ask before you sign anything.
Do you need planning permission?
Most rear-facing Velux and rear dormer conversions fall under Permitted Development, which means no planning application. There are limits: 40 cubic metres of added volume on a terraced house, 50 on a semi or detached, nothing forward of the original roof slope facing a highway, and materials that match the existing house.
Mansard conversions and many hip-to-gable conversions usually require full planning permission, particularly in conservation areas. Flats do not have Permitted Development rights at all, so a loft conversion on a flat always needs an application.
Every loft conversion, whatever the type, needs Building Regulations approval covering fire safety, the staircase, insulation and ventilation. Permitted Development is not an exemption from that, and the two are often confused.
Is a loft conversion worth it?
A well-executed loft conversion can add roughly 20 to 24 percent to a property's value. In many London and Essex postcodes the value added exceeds the build cost, particularly where the conversion adds a fourth or fifth bedroom with an en-suite.
The other argument is one people forget until they compare options. A loft conversion keeps your garden. A ground floor extension takes it. On a standard terrace plot in North London, where outside space carries real weight at resale, that is not a small consideration.
How to compare loft conversion quotes properly
Three quotes that look wildly different are usually pricing different jobs, not the same job at different rates.
Check whether VAT is included or added. Check whether the figure includes drawings and structural calculations or assumes you have them already. Check whether the staircase is in the price. Check what the flooring and decoration specification actually is. Then ask each builder for a payment schedule tied to stages of work rather than dates, and ask what happens to the price if the roof opens up and the structure is not what was assumed.
The cheapest quote is often the one with the most sitting outside it.
Loft conversion cost: Frequently asked questions
How much does a loft conversion cost in London?
Between £40,000 and £95,000 in 2026, depending on the type. A Velux starts around £40,000, a dormer runs £50,000 to £65,000, and a mansard £70,000 to £95,000 or more.
What is the cheapest type of loft conversion?
A Velux, or rooflight, conversion. It adds windows to the existing roof slope without changing the roofline, so it needs the least structural work. It does require good existing head height.
Do I need planning permission for a loft conversion?
Often not. Many rear Velux and dormer conversions fall under Permitted Development. Mansard and hip-to-gable conversions usually need planning permission. Every conversion needs Building Regulations approval.
How long does a loft conversion take?
The build runs from 4 to 6 weeks for a Velux to 10 to 16 weeks for a mansard. Add time for design, drawings and any planning application before work starts on site.
Does a loft conversion add value?
Yes, typically 20 to 24 percent, and in many London and Essex areas the value added is more than the cost of the work.
Planning a loft conversion in London or Essex?
Get a clear, fixed quote from Mammoth Builds. We handle design, structure, Building Regulations and the full build. Request your free consultation today.
