Dormer, hip-to-gable and mansard loft conversion drawings for Cosham homes — prepared to Portsmouth City Council's design guidance, delivered in days and revised free until your decision lands, with a free approval-odds score 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.
Cosham's roofs set the brief. The borough is built largely from interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing, and its lofts sit under hipped roofs on the interwar semis and pitched tile roofs on the older cottages — the geometry that decides whether a rear dormer, a hip-to-gable or a mansard gives you the headroom you want. Portsmouth City Council assesses loft proposals against the Portsmouth Local Plan and the council's Houses in Multiple Occupation SPD, and its case officers read each roof against the pair next door and the wider street before they read the plans.
That makes drawing quality decisive. We measure ridge and eaves heights, roof pitch and the existing stair, calculate the added volume against the permitted-development limits exactly, and show the workings on the drawings so Portsmouth City Council can verify them. The interwar semis are read as matched pairs, so hip-to-gable and roof changes are judged against the symmetry of the neighbour, so we resolve it on the page rather than leave it for the case officer to raise.
Cosham also carries protected streets — Old Wymering and Hilsea Lines among them. 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 check your address against those designations before a line is drawn.
Many Cosham loft conversions proceed without planning permission — a rear dormer within the permitted-development volume limits (40 cubic metres on a terrace, 50 on a semi or detached house) is usually lawful. You will normally need a full application to Portsmouth City Council if any of these apply:
We draw the roof line, the ridge height and the neighbour's daylight on the page — the relationships Portsmouth City Council's officers check first — 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, pulled from official planning statistics and updated 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. Professionally prepared loft drawings anticipate those pressure points before submission, which is why complete, policy-literate packs are decided faster and refused less often than DIY applications.
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 recurring loft is the rear dormer — a box across the back roof slope that turns a cramped attic into a full-height double bedroom and en-suite. On hipped roofs on the interwar semis, that is usually the most efficient use of the roof; where the stock is pitched tile roofs on the older cottages, a hip-to-gable followed by a rear dormer unlocks far more space, though it needs a full application.
The historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. We draw the dormer inside the volume allowance, keep the flat-roof build-up below the ridge, and set the face back from the eaves as the rules require — so the pack answers Portsmouth City Council's questions before they are asked. Where permitted-development rights are exhausted or removed, we prepare a householder application composed to the borough's roof-alteration guidance instead.
We draw loft conversions on every street in the borough. A few of the neighbourhoods where Cosham roofs come up most often:
Loft conversion drawings for Old Wymering's the historic village core around the medieval manor and church — measured, volume-checked and drawn to Portsmouth City Council's guidance.
Loft conversion drawings for Hilsea Lines's the 19th-century ramparts and moat guarding the island's neck — measured, volume-checked and drawn to Portsmouth City Council's guidance.
Loft conversion drawings for Cosham High Street's the mainland town centre fringed by interwar housing — measured, volume-checked and drawn to Portsmouth City Council's guidance.
Loft conversion drawings for Highbury's the planned 1930s estate of semis below Portsdown Hill — measured, volume-checked and drawn to Portsmouth City Council's guidance.
Loft conversion drawings for Drayton's interwar and postwar suburban housing to the east — measured, volume-checked and drawn to Portsmouth City Council's guidance.
Loft conversion drawings for Portsdown fringe's houses climbing toward the chalk ridge and its forts — measured, volume-checked and drawn to Portsmouth City Council's guidance.
The right loft in Cosham depends on your roof. This is how the main types compare on planning route and the space they add.
| Loft type | Permitted development? | Space & headroom | What Portsmouth City Council checks |
|---|---|---|---|
| Rear dormer | Usually yes, within volume limits | Full-height double room at the back | Volume, set-back from eaves, ridge height |
| Hip-to-gable | No — full application | Extends the whole side of the roof | Roof bulk and the symmetry of the pair |
| Mansard | No — full application | Maximum floor area across the roof | Street character, especially in conservation areas |
| Rooflights only | Usually yes (Class C) | Light and headroom, no extra footprint | Whether rooflights sit within the existing slope |
Route depends on your street — conservation areas such as Old Wymering and Hilsea Lines and Article 4 directions can remove permitted-development rights.
A fixed process built around how Portsmouth City Council validates and decides householder loft applications — most drawings are submission-ready within seven days of survey.
Tell us your Cosham postcode and roof type. We confirm your conservation and Article 4 status, 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, joist levels and the existing stair — the numbers the volume calculation and head heights depend on.
Chartered designers develop the dormer, hip-to-gable or mansard to your brief, drawn inside the Class B limits or 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 dormer within 40 cubic metres on a terrace, or 50 on a semi-detached or detached house, set back from the eaves and no higher than the ridge, is usually permitted development. In Cosham the main exceptions are hip-to-gable conversions, dormers visible from the highway, and homes in conservation areas such as Old Wymering and Hilsea Lines or on streets with an Article 4 direction. Portsmouth City Council is the decision-maker.
Generally no. In conservation areas including Old Wymering and Hilsea Lines, Class B permitted-development rights for lofts are removed, so a rear dormer that would be lawful elsewhere needs a full application to Portsmouth City Council. 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 check your exact address before drawing.
On hipped roofs on the interwar semis, common across Cosham, a rear dormer is usually the most efficient conversion and often permitted development. Where the roof is pitched tile roofs on the older cottages, a hip-to-gable followed by a rear dormer gains far more usable space but needs planning permission. Our survey confirms which your roof supports before we draw. Portsmouth City Council is the decision-maker.
A householder planning application in Cosham runs to a statutory eight-week target, and a lawful development certificate to eight weeks as well, though validation can add time if the pack is incomplete. We submit validation-complete drawings and manage case-officer queries to keep the decision on track. Portsmouth City Council is the decision-maker.
If your dormer is permitted development, a lawful development certificate from Portsmouth City Council is council-stamped proof of that — worth having, because every buyer's solicitor will ask for it when you sell. We prepare and submit the certificate application with the same drawing pack. This applies to your Cosham address specifically.
Every roof is different, so we quote each Cosham project individually — a fixed, no-obligation price back within one working day. The quote covers the full drawing pack with free revisions until Portsmouth City Council approves, and we tell you upfront whether your loft qualifies as permitted development. 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.