Rear dormer drawings for Cosham roofs — the box across the back slope that turns a low attic into a full-height room, drawn to Portsmouth City Council's guidance and the volume limits, delivered in days and revised free until your decision, with a free approval-odds check first.
Portsmouth City Council granted 88% of the householder planning decisions it made in its latest published year — 1 point above the 87% England-wide householder average. The interwar semis are read as matched pairs, so hip-to-gable and roof changes are judged against the symmetry of the neighbour, which is where a policy-literate drawing pack earns its keep. Its dominant interwar (1918–1939) and postwar (1945–1970) housing stock sets the design brief on most streets.
The permitted-development ceilings are fixed: 40m³ of new roof volume for a terraced loft and 50m³ for a semi or detached house under Class B, plus a 3m single-storey rear extension on a terrace or 4m on a detached house under Class A. Cosham carries at least one Article 4 direction, and conservation areas including Old Wymering and Hilsea Lines remove those rights outright, so we confirm your exact address before a line is drawn. A validated householder application in Cosham is determined against an 8-week statutory target.
Old Wymering (the historic village core around the medieval manor and church), Hilsea Lines (the 19th-century ramparts and moat guarding the island's neck), Cosham High Street (the mainland town centre fringed by interwar housing), Highbury (the planned 1930s estate of semis below Portsdown Hill), Drayton (interwar and postwar suburban housing to the east) and Portsdown fringe (houses climbing toward the chalk ridge and its forts) — these are the Cosham pockets our drawings cover most. Heritage designations here include Old Wymering and Hilsea Lines. Portsmouth City Council weighs each proposal against the Portsmouth Local Plan and the council's Houses in Multiple Occupation SPD, where the historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. Cosham holds pockets of converted flats and shared houses around the town centre that sit outside householder permitted-development rights.. On roofs the borough is hipped roofs on the interwar semis and pitched tile roofs on the older cottages, over interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing.
The box dormer is the workhorse loft, and most of Cosham is built for it — the borough runs to interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing under hipped roofs on the interwar semis and pitched tile roofs on the older cottages, and a flat-roofed box across the rear slope is the cheapest cubic metre of full-height space those roofs give up. Portsmouth City Council tests the dormer against the Portsmouth Local Plan and the council's Houses in Multiple Occupation SPD, and because permitted development turns on a number, the volume you propose against the volume you have left is the first thing an officer or a certificate checks.
That is why the arithmetic goes on the drawings. We calculate the box against the 40- or 50-cubic-metre allowance, count any earlier roof enlargement into the total, hold the flat roof below the ridge and set the face back off the eaves so the dormer reads as subordinate. The interwar semis are read as matched pairs, so hip-to-gable and roof changes are judged against the symmetry of the neighbour, so the cheeks, the window line and the material are drawn to answer it rather than invite a query.
Where Cosham's streets are protected — Old Wymering and Hilsea Lines among them — the Class B dormer right is switched off and even a modest rear box needs consent. Portsmouth's Article 4 direction covers the whole city including the mainland, so even in the more suburban Cosham and Wymering area a change of a family house to a small HMO needs planning permission. We confirm the designation before the box is sized.
Many rear dormers in Cosham are permitted development, but the volume and position decide it. You will normally need a full application to Portsmouth City Council if any of these apply:
We draw the box within its volume allowance, the flat roof below the ridge and the face set back off the eaves — the three things a dormer stands or falls on — and submit a validation-complete pack, whether that is a lawful development certificate or a full householder application.
Below is Portsmouth City Council's real householder decision record, drawn from official planning statistics and refreshed as new data lands. Portsmouth approves a solid share of householder schemes in Cosham, where interwar symmetry, the Wymering and Hilsea Lines heritage and Portsdown Hill views shape most refusals. A dormer is decided on volume and set-back, so a pack that shows the calculation and the eaves detail is cleared faster and refused less often than a DIY submission.
Source: MHCLG planning application statistics (householder decisions, England, 1 April 2025 - 31 March 2026). Compare every borough on our London approval-rate league table.
Tell us about your project and get a fixed, no-obligation quote back within one working day. If you don't need planning permission at all, we'll tell you that too — for free.
Across Old Wymering, Hilsea Lines and Cosham High Street, the dormer that goes through is the subordinate rear box: flat roof kept low, cheeks clad to recede, side windows obscure-glazed against overlooking, and the whole thing set back so the original eaves line still reads from the garden. On hipped roofs on the interwar semis, this returns a full-height double and en-suite from what was standing-room-only, usually inside the permitted-development volume.
The historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. We keep the box off the ridge, match the cheek material to the roof, and where the volume is already spent — or the street is designated — draw the dormer as a householder application composed to Portsmouth City Council's roof-alteration guidance rather than push a certificate that would be refused.
We draw rear dormers on the terraced and semi-detached streets across the borough. A few of the neighbourhoods where Cosham dormers come up most often:
Dormer loft drawings for Old Wymering's the historic village core around the medieval manor and church — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Dormer loft drawings for Hilsea Lines's the 19th-century ramparts and moat guarding the island's neck — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Dormer loft drawings for Cosham High Street's the mainland town centre fringed by interwar housing — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Dormer loft drawings for Highbury's the planned 1930s estate of semis below Portsdown Hill — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Dormer loft drawings for Drayton's interwar and postwar suburban housing to the east — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Dormer loft drawings for Portsdown fringe's houses climbing toward the chalk ridge and its forts — volume-checked, set-back-detailed and drawn to Portsmouth City Council's guidance.
Whether a dormer in Cosham needs permission comes down to where it sits and how big the box is. This is how the cases compare.
| Dormer type | Permitted development? | What it gains | What Portsmouth City Council checks |
|---|---|---|---|
| Rear box, within volume | Usually yes | Full-height room across the back | Volume total, 20cm eaves set-back |
| Rear box, over volume limit | No — full application | A larger room | How far over the allowance, bulk |
| Front or side dormer | No — full application | Light to the front rooms | Visibility from the street, character |
| Dormer in a conservation area | No — full application | Same space, extra scrutiny | Roof form and street appearance |
In conservation areas such as Old Wymering and Hilsea Lines, Class B dormer rights are removed and every dormer needs a full application.
A fixed process built around how Portsmouth City Council validates and decides dormer applications — most drawings are submission-ready within seven days of survey.
Send us your Cosham postcode and roof type. We confirm your conservation and Article 4 status, work out how much permitted-development volume you have left, and score your odds against live Portsmouth City Council decisions.
We measure ridge and eaves heights, roof pitch and the existing loft floor — the numbers the volume calculation, the set-back and the head heights depend on.
Chartered designers develop the box to your brief, drawn inside the Class B volume limits or, where those are exhausted or removed, to Portsmouth City Council's roof-alteration guidance.
We submit the certificate or application, manage validation and case-officer queries, and revise the drawings free until Portsmouth City Council issues its decision.
Often not. A rear box dormer within 40 cubic metres on a terrace, or 50 on a semi or detached house, set back at least 20 centimetres from the eaves and kept below the ridge, is usually permitted development. In Cosham you will need a full application to Portsmouth City Council for a front or side dormer, a box that exceeds the volume, and any dormer in a conservation area such as Old Wymering and Hilsea Lines, where those rights are removed.
The permitted-development limit is a volume, not a footprint: 40 cubic metres of added roof space on a terraced house and 50 on a semi-detached or detached one, and any earlier loft enlargement counts against that total. In Cosham the box must also sit below the ridge and back from the eaves. We measure your roof and calculate the exact volume you have left before drawing. Portsmouth City Council is the decision-maker.
Not under permitted development. A dormer on the front roof slope, or one facing a highway, always needs a full application to Portsmouth City Council, because it changes how the house reads from the street. Rear box dormers are far more likely to be approved — or to be permitted development outright — which is why almost every Cosham dormer we draw sits on the back slope.
Often, on a terrace or semi. If the dormer or the loft steelwork meets the party wall with your neighbour, the Party Wall etc. Act applies and notice must be served before work starts — separately from Portsmouth City Council's planning decision. We flag the party-wall junction on the drawings so a surveyor can be appointed early and the build is not held up. This applies to your Cosham address specifically.
If your dormer is permitted development, a lawful development certificate from Portsmouth City Council is council-stamped proof of it — worth holding, because a buyer's solicitor will ask for it when you sell. We prepare and submit the certificate with the same measured drawing pack that shows the box within its volume allowance. This applies to your Cosham address specifically.
We price each Cosham dormer individually, because the roof pitch and the volume left vary house to house, and send a fixed, no-obligation quote within one working day. It covers the full drawing pack with free revisions until Portsmouth City Council approves, and we confirm at the outset whether your box is permitted development or needs an application. Call 020 7864 7245 to start.
Answer five quick questions and get a free planning score for your exact address — instantly. Then, if you want the full picture, unlock the detailed analysis: your council's live approval rates, local precedents, constraints on your street and a step-by-step route to permission.
Same fixed quotes, same fast drawings — wherever your project is.