L Shaped Dormer Conversions
Two dormers joined over the main roof and the rear outrigger, on Victorian and Edwardian houses.
Fig. 01
An L shaped dormer is the most floor area a terraced house can gain without taking any garden, because it uses the roof over the back addition as well as the main roof. It is also the conversion with the most demanding detail in it, and that detail is the corner where the two dormers meet.
What an L shaped dormer is, and which houses have one
Two dormers joined in an L, one over the main rear roof and one over the outrigger.
Most Victorian and Edwardian terraces were built as a main block two rooms deep with a narrower outrigger running back from it, sometimes called a back addition or a rear wing. It holds the original scullery below and a small bedroom or bathroom above, and it has its own roof, lower than the main one and often at a different pitch.
A plain rear dormer works on the main roof alone. An L shaped dormer builds a second dormer over the outrigger and joins the two, so the new floor turns the corner and follows the shape of the house. From the garden it reads as one dormer bent round a right angle. From inside it is the difference between a room and a small suite.
The house has to have the outrigger for any of it to be possible, and that is the whole test. In Colchester it means the Victorian and Edwardian terraces, plus a number of Edwardian and early interwar semis built with a back addition rather than a square plan. A 1930s hipped semi has no outrigger, so the job there is a hip to gable instead.
What the outrigger roof is matters too. A pitched roof with its own small ridge is the straightforward case. A lean-to sloping off the main rear wall is more awkward, because the geometry at the junction changes, and a flat roofed outrigger is generally a rebuild rather than a conversion.
What it gains over a plain rear dormer
Roughly half as much again, and it is the half that makes the layout work.
A rear dormer on the main roof gives you one good double bedroom and, if the width allows, a tight en suite off one end. The L shape adds the outrigger, which is narrow but useful. On a typical Colchester terrace it is around 2.4 to 3 metres wide externally, under 2.5 metres inside, which suits an en suite, a dressing area or a study and takes those functions out of the main space.
It also gives the staircase somewhere to arrive, which a single dormer on a terrace rarely does.
| Plain rear dormer | L shaped dormer | |
|---|---|---|
| Usable floor area added | Around 18 to 25 m² | Around 28 to 38 m² |
| Typical result | Double bedroom, tight en suite | Double bedroom, en suite, landing |
| Terraced volume allowance used | Part of the 40 m³ | Most or all of the 40 m³ |
| Valley junctions to weather | None | One, and it is the whole job |
| Indicative build cost | £1,500 to £2,200 per m² | £1,800 to £2,500 per m² |
| On site | 6 to 10 weeks | 8 to 12 weeks |
Will you need planning permission?
Four questions, an honest answer, and nothing sent anywhere. This runs entirely in your browser.
What are you extending?
Permitted development rights for extensions apply to houses. Flats and maisonettes are a different route entirely.
Is the property listed?
Listed building consent is a separate decision from planning permission, and work without it is a criminal offence rather than a planning irregularity.
Has the house been extended before?
Depth is measured from the ORIGINAL rear wall, so a previous owner's extension has already used part of your allowance. This catches almost everybody.
Is it in a conservation area?
Colchester has a number of them, including the historic core, the Dutch Quarter, part of Lexden and several village centres. The boundaries rarely follow the roads you would expect.
Answer the four questions for an indication
Whatever the answer, Building Regulations approval is always needed. We check the planning history, the designation and the deeds on your address before anyone draws anything, because it decides the design rather than the other way round.
This is an indication based on four questions, not advice on your property. Planning rules change and the position depends on your specific address, its planning history and its designation.
The valley junction, and why it is the hardest part of the job
Two flat roofs, two sets of cheeks and two existing slopes all arrive at one internal corner.
Everything difficult about an L shaped dormer happens within about a metre and a half of one point. The main dormer has a flat roof and vertical cheeks. The outrigger dormer has its own flat roof at a lower level and its own cheeks. Where they meet there is an internal corner, a change of level, and below it the existing slopes running down into a valley. Water off a large area of roof is funnelled into that corner and it has to leave.
Getting it right starts with the falls, and they are designed before anything is cut. Both flat roofs need a positive fall, and the two have to agree with each other so water crosses the corner rather than sitting in it. That is set out with firrings on the drawings, not judged on the deck by eye. Ponding is the commonest thing we find on an existing L shaped dormer giving trouble, and it is nearly always because the corner ended up the low point of both roofs at once.
The covering matters as much as the fall. The junction wants forming in one continuous piece, dressed round the corner and up the abutments, with no seam or lap sitting where the water concentrates. Where a mitre is unavoidable it belongs up on the slope, away from the corner. Upstands need proper height, support behind them, and a termination into the existing structure rather than a bead of mastic at a change of direction.
Below the dormers, the existing valley between the two roofs usually has to be rebuilt, because the new cheeks land on it. That means soakers and a secret gutter where cheek meets tiling, not a fillet of mortar. On a terrace there is frequently a party parapet running up between the houses as well, so the water passes another abutment on its way out.
If an L shaped dormer leaks, it will leak at that corner, and it will usually do it two or three winters after the job finished rather than immediately. Ask whoever quotes you how they intend to form the junction and where the water leaves. A clear answer tells you more about a loft firm than anything else you can ask.
Working out the 40 cubic metres
The allowance is a volume, and an L shaped dormer eats most of it.
Loft conversions have their own permitted development class and the allowance is a volume of additional roof space: broadly 40 cubic metres for a terraced house, 50 for a semi detached or detached one. The arithmetic decides the route, so it gets done first.
What counts is the volume added above the original roof surfaces, not the size of room you end up with. As a rough feel, a dormer across most of the rear slope of a terrace main roof, say 4.5 metres wide running 3.5 metres up the slope, comes out somewhere around 25 to 30 cubic metres. The outrigger dormer adds another 8 to 14 depending on its width, which puts a good number of houses at or past the terraced allowance.
The allowance is cumulative and includes anything previously added to the roof by any owner, so a small bathroom dormer already sitting on the outrigger has spent part of it. Rooflights do not count, because they do not project.
So these jobs split two ways. Where the house is wider, or the outrigger short, the design fits inside 40 cubic metres and it is permitted development. Where it does not, the choice is to shrink the design or to apply for permission and build the space the house actually wants. Building it and finding out afterwards is the one option with no good ending.
Want this checked on your actual address?
Planning history, designation and deeds. We look at all three before anyone draws anything, and it costs you nothing.
The conditions that come with the volume
Staying inside the volume is necessary but it is not sufficient.
Nothing may sit higher than the highest part of the existing roof, which here is the main ridge. The outrigger dormer normally sits well below it, though where the outrigger is built up close to full height the level wants checking rather than assuming.
Neither dormer may go beyond the plane of the roof slope on the principal elevation facing a highway, which on a terrace means the front. Rear dormers must be set back from the eaves, measured along the roof plane, rather than running out to them. On the main roof that is easy. On the outrigger it costs you, because it is narrow and its eaves are close on both sides, so the set back takes a real slice out of a tight width and often decides whether that arm is a room or a corridor.
Materials must be similar in appearance to the existing house. On a rear elevation that is judged from neighbouring gardens rather than the street, but it still applies.
Side facing windows in the new roof space must be obscure glazed and non opening below 1.7 metres from the finished floor. That matters more here than usual, because the outrigger arm has two side elevations and often no rear one. It also collides with the fire escape requirement further down this page.
What removes permitted development in Colchester
Conservation areas are the usual reason an L shaped dormer becomes an application.
Permitted development is removed outright for flats and maisonettes, and where an Article 4 direction applies. Listing does not remove it, but it adds listed building consent as a separate requirement. In a conservation area a rear dormer generally loses permitted status altogether and becomes a full planning application.
That matters here more than it would elsewhere, because Colchester’s conservation area designations cover a good deal of the older housing and the historic core in particular. The Dutch Quarter and the streets around Maidenburgh Street and St Helen’s Lane sit within designated areas, and a proportion of that stock is listed as well, which brings listed building consent in on top of planning permission. Terraced housing further out is more mixed, and a boundary can run along one side of a street and not the other.
None of that means the answer is no. Rear dormers in conservation areas are applied for and decided all the time. It does mean a longer front end and a decision that belongs to the council rather than to anybody quoting you.
Where the conversion is permitted development, apply for a Lawful Development Certificate. It is not planning permission and it is not optional in any practical sense: it is the council confirming in writing that what you built did not need permission. A dormer sitting close to the 40 cubic metre line is exactly what a buyer’s solicitor will query, and the certificate plus the Building Control completion certificate are what answer it.
The same wall, before and after
Drag to reveal. A real job in the Colchester area, photographed as it went. No staging and no borrowed images.
During
Finished
Two roofs, two head heights, and the step between them
The outrigger almost never matches the main roof, and sometimes the floors do not line up either.
Measure both roofs separately, from the top of the ceiling joists to the underside of the ridge. On a Victorian terrace the main roof is usually steep and generous, frequently 2.6 metres or more. The outrigger is a different proposition: lower ridge, narrower span, sometimes a shallower pitch, so it can be well under 2 metres before anything is built. The dormer creates the height, but the outrigger dormer ends up taller in proportion to its roof, which is a volume question as well as a design one.
The other surprise is the floor level. Many terraces with a back addition had the outrigger floors set lower than the main house, which is where the half landing between them comes from. At loft level the wall plates can be at different heights, and the new structure sometimes cannot bring both sides to one level without losing head height in the main room. The result is a step of a couple of hundred millimetres between the two parts of the new floor.
A step inside a bedroom is liveable. A step on the landing where the stairs arrive is a different matter, because that is an escape route and it has to be safe in the dark. Where it cannot be designed out it wants to be a proper single riser in a sensible place with a light switched beside it, not an accident discovered on site.
Structure, and the walls it lands on
New floors on steels, two dormer openings trimmed, and one corner carrying both.
The timbers currently spanning your loft were sized to hold up a ceiling. The conversion introduces a new floor of deeper joists, generally carried on steel beams at each end, sitting above the existing ceiling joists rather than replacing them. A terrace of this age will have a cut roof rather than trusses.
On a mid terrace the main floor beams almost always bear into the party walls on both sides. That means cutting pockets into two shared walls, with padstones spreading a concentrated load onto masonry that has been carrying a distributed one since the 1890s, and the bearings get checked by the structural engineer rather than assumed. The chimney breasts in those walls are a separate question again.
The outrigger is where it gets interesting. Its flank walls are frequently only half a brick thick, because they were never asked to carry much. A steel cannot simply be dropped onto a wall like that, so the answer is usually a pier, a spreader, or bringing the load back into the main house instead. All of those work, and all of them need knowing about before the price is fixed, which is why this job wants a proper look in the roof rather than a driveway estimate.
Then there are two dormer openings to trim, and the corner where the dormers meet, carrying the ends of both flat roof structures and the convergence of both sets of cheeks. That corner is a designed structural element with a designed weathering detail on top of it, and the two are drawn together.
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Trust on a building job should be verifiable, not asserted. Every one of these can be confirmed without taking our word for it.
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Real accountability
We notify Colchester City Council Building Control and manage the inspections through to your completion certificate.
Party wall, with a neighbour on each side
A mid terrace means two sets of notices, and sometimes a third wall in the outrigger.
Cutting a beam pocket into a party wall is notifiable work to a party structure under section 2 of the Party Wall etc. Act 1996. So is raising, cutting into or weathering against a party wall or a party parapet, all of which an L shaped dormer tends to do. The notice is served by you as the building owner and it carries a two month period before work can start.
Section 1 applies where new work is built up to or astride the boundary line, which arises where a dormer cheek sits on the line of junction with a neighbouring outrigger, and that notice runs one month. Section 6 covers excavation near a neighbour’s foundations and normally has nothing to do with a loft, but it returns where a load has to come down through the outrigger onto a new pad or a strengthened footing.
The particular reality of a terrace is that you serve on two adjoining owners rather than one, and where outriggers touch there can be more than one relationship on the same side. Each can consent, dissent, or do nothing, and doing nothing counts as dissent. Each dissent means surveyors and an award, and you pay for it.
A schedule of condition before work starts prevents an argument eighteen months later about a crack that was already there. Tell both neighbours yourself, with the drawings, before a legal notice lands on the mat.
The staircase and the narrow landing
On a terrace the stair is usually the constraint, and the L shape is what rescues it.
Part K sets a maximum pitch of 42 degrees, minimum going and maximum rise, handrails, and 2 metres of headroom measured vertically above the pitch line and at the top. On a narrow terrace landing, often barely a metre wide with three doors off it, those requirements are tight.
The usual position is over the existing flight, which keeps the circulation stacked and does not cost a bedroom. Whether it works depends on the landing length and the roof structure the stair rises through, so the headroom is checked at survey with a tape.
Where the stair lands is where the L shape earns its keep. A single rear dormer usually leaves the arrival inside the new bedroom or on a landing you cannot pass someone on. Turning the corner into the outrigger gives the top of the flight somewhere to arrive and lets the bedroom and en suite doors come off it properly. The landing below still has to work afterwards, and a new flight that turns it into a corridor with four doors fouling each other has solved one problem by creating another.
What might it cost?
A realistic range rather than a single number, with what it excludes shown as prominently as what it includes.
£44k to £60k
- VAT
- Kitchen and appliances
- Flooring
- Decoration
- Drawings and calculations
- Planning and Building Control fees
- Party wall surveyor
- Service diversions
Indicative only. The biggest variable around Colchester is foundation depth, because most of the ground is shrinkable clay and mature trees are common. We price the job after seeing it.
Get a real price
Fire, and the window you are not allowed to have
A third storey changes the escape strategy for the whole house.
Once the house has three storeys the windows stop being an escape route, so the staircase becomes one and it has to be protected from the top floor to the final exit door. In practice that means fire doors on habitable rooms opening onto the stairwell on every floor, 30 minutes of fire resistance to the walls and ceilings enclosing it, interlinked mains powered smoke alarms on every level, and 30 minutes fire resistance to the new floor from below.
Terraces bring a specific complication. A great many had the front and back rooms knocked into one years ago, and the stairs come down into that open space, which is exactly what a protected stairwell cannot tolerate. The alternatives are a sprinkler or misting system, or an agreed escape arrangement, both settled with Building Control before work starts rather than discovered at first inspection.
Every habitable room in the new space needs an escape window: at least 0.33 square metres of clear opening, no less than 450mm in either dimension, with the bottom of the opening between 800mm and 1100mm above the floor.
Here is where it collides with the planning conditions. If the outrigger arm is a habitable room it needs an escape window, and the elevations available to it are side facing. A side facing window in permitted development roof space must be obscure glazed and non opening below 1.7 metres, and an escape window with an 1100mm sill is below that. Both cannot be met in the same opening. The resolutions are to make that arm an en suite or dressing room, which needs no escape window, to bring a window into the rear face of the outrigger dormer, or to accept that the design is a planning application. It is a decision taken at the start, not a detail sorted later.
Insulation, and why a cold roof fails at the corner
Two flat roofs meeting at a right angle is exactly where cross ventilation stops working.
A flat roof can be built cold, with insulation between the joists and a ventilated void above, or warm, with the insulation above the deck and no void at all. Both are legitimate on a simple rectangular dormer.
On an L shaped dormer the cold version has a problem specific to the shape. A ventilated void relies on air crossing from one side to the other, and at the internal corner the two roofs block each other’s path. The corner becomes a dead pocket of humid air against a cold deck, condensation forms there, and it drips and eventually rots the structure that was just installed. It is a slow failure that presents as a leak, which is why it tends to get chased as one.
A warm roof avoids it, because there is no void to ventilate and the deck stays above the dew point. It adds a little height, which gets checked against the ridge condition and the volume sum, and it needs the vapour control layer taken continuously round the corner and up the cheeks.
Summer heat is the other half. A large flat roof over a bedroom facing south or west gets hot, so insulation depth beyond the minimum, solar control glass and openable windows high enough to let warm air out all matter, and none of it is required of you, because the overheating rules cover new dwellings rather than a conversion of the roof you already have. The one advantage of the L shape is that the two arms face different ways, so a window in each gives real cross ventilation.
Get a real number, not a range.
We come out, look at the ground, the access and the structure, and price the job we have actually seen. The survey and the quotation are free.
Access on a terrace, and how long it takes
Everything comes through the house, and the scaffold takes the garden.
A mid terrace has no side access, so every steel, joist, board and skip load of stripped tiles travels through the front door, along the hall and up the stairs. Protection to the floors and a sealed working route are not niceties on this job, they are the difference between living there and not.
Scaffolding goes up at the rear over a garden that is generally small, and an L shaped dormer needs it to the main elevation and round the outrigger as well, so most of the garden is gone for the duration. Getting it in is its own question: sometimes it comes through the house in sections, sometimes it needs the neighbour’s agreement to cross their land, and the Party Wall Act gives rights of entry for notifiable work rather than a general licence to occupy a garden. A skip on the road needs a permit from the highway authority, and on the older terraced streets that means a bay suspension too.
On timing, the front end is either a Lawful Development Certificate or a full application at the statutory eight weeks from validation, with Building Regulations full plans approval alongside and party wall notices running their two months from service. On site, an L shaped dormer is commonly eight to twelve weeks. The roof is open under temporary sheeting for part of that, and the most intrusive week is the staircase, because that happens inside the house.
What it costs
Indicative and current at the review date below, not a quotation.
An L shaped dormer conversion generally runs £1,800 to £2,500 per square metre of finished floor area. It sits above a plain rear dormer for good reasons: two roofs to open, two sets of cheeks, the junction, more structure and more scaffold.
That includes the structural steels and new floors to both arms, roof alterations and all the weathering including the valley junction, insulation, the staircase, fire doors and the protected stairwell works, dormer windows and rooflights, plastering, first and second fix carpentry, electrics and plumbing to a standard specification, and making good.
It excludes VAT, en suite sanitaryware and tiling, fitted wardrobes, flooring, decoration, professional fees for drawings and structural calculations, planning and Building Control fees, party wall surveyor fees, and anything genuinely unforeseeable once the roof is open.
Two lines deserve singling out on this house type. Party wall fees can arrive twice, because a mid terrace has an adjoining owner on each side and either or both may dissent. And where a load has to come down through the outrigger, the structural solution is more involved than a simple pair of beams. Both belong in the quotation as identified possibilities rather than as a surprise in week three.
What moves the figure most is the state of the outrigger walls, what the existing stairwell needs for fire protection, and whether the design fits inside permitted development.
Fig. 01
What actually happens, in order
Three of these eight stages happen before anybody digs anything. Get the order wrong and you pay to redo work.
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Survey
We look at the ground, the trees, the access, the existing structure and where the drains actually run.
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Drawings
A technician produces existing and proposed plans and elevations to a standard the council will accept.
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Permissions
Permitted development, prior approval or a full application. We submit it and handle the correspondence.
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Structure
The engineer calculates the beams, the padstones and the foundation depth for your ground.
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Groundworks
Dig, concrete and drainage. The messiest phase and the one that decides whether the rest stands still.
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Structure up
Walls, steel into the opening, roof on, windows and doors in. The day the back wall comes out is planned with you.
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Fit out
First fix, plaster, second fix. From here it stops being a building site and starts being a room.
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Certificate
Final inspection and your Building Control completion certificate. Keep it with the deeds.
Questions we get asked
Will an L shaped dormer definitely be permitted development?
It depends on the volume sum and on where the house is. Plenty fit inside the 40 cubic metre terraced allowance and plenty do not, and a conservation area removes the option regardless. We work the volume out from measurements before saying anything about the route.
Does the outrigger part have to be an en suite?
No, but it frequently ends up that way, partly because the width suits it and partly because a room with no escape window cannot be a bedroom. A study, dressing room or bathroom all work in that arm. A second bedroom needs the window question solving first.
Can the two dormers be built at different times?
In theory, and it is a poor idea. The junction is a single detail formed in one operation, so building the main dormer first and cutting into it later means opening up a corner that was already weathered. It also means scaffolding twice.
Do both my neighbours have to agree?
They do not have to agree, and disagreement does not stop the work. If an adjoining owner dissents, surveyors are appointed and an award records how the work proceeds and the condition of their property beforehand. What you cannot do is skip the notice.
Which Colchester housing has the outrigger
Late Victorian and Edwardian terraces, and a few Edwardian semis.
New Town is the obvious one. The grid south of the centre, Wellesley Road, Wimpole Road, Papillon Road, Artillery Street and the roads off Military Road, is largely late Victorian and Edwardian terraced housing with back additions and tall steeply pitched main roofs. It is close to ideal stock: real head height in the main roof, a proper outrigger, and gardens long enough to take a scaffold.
The terraces off Maldon Road and around Butt Road are similar in age and plan. Around the Hythe, along Magdalen Street and Barrack Street, there is more variation and some rebuilt stock, so those roofs want looking at individually.
The historic core and the Dutch Quarter, around Stockwell Street, Maidenburgh Street and St Helen’s Lane, hold older and frequently listed property inside designated conservation areas. There are terraces there with outriggers, and the route is a planning application, sometimes with listed building consent alongside, rather than permitted development.
Edwardian and early interwar semis along parts of Lexden Road, Beaconsfield Road and Creffield Road were often built with a back addition, so the L shaped option exists on some of them. The later 1930s hipped semis in Prettygate and Shrub End were not, and those houses want a hip to gable instead.
How we work
One contract, one contact, and the paperwork is ours to deal with.
Our team has 25+ years in the trade and comes out. On an L shaped dormer the survey measures both roofs separately, looks at the outrigger walls and what they can carry, works out the volume, checks where a compliant staircase can land, and looks at what the existing stairwell needs for fire protection.
Most people who call us already have drawings and permission and want a builder to price the work and build it. That is the ordinary job here, and we are glad to work to somebody else’s plans. Where the drawings do not exist yet, we bring in the technician who draws it and the structural engineer who sizes the steels, we deal with the planning route or the Lawful Development Certificate, we serve the party wall notices on both sides, and we notify Colchester City Council Building Control and manage the inspections through to your completion certificate.
Probuild Extensions Colchester is the extensions arm of Essex Probuild Ltd, company number 15979928. We carry £2m public liability cover and everything is built to current Building Regulations. We are based in West Bergholt, about three miles north west of Colchester, and we work across the city and roughly thirty miles around. The survey and the quotation cost nothing.
Last reviewed August 2026. Planning rules and Building Regulations change, and your property may differ, so treat this as a guide rather than advice on your specific job.