Single-storey, double-storey, rear and side extension drawings for Cosham homes — measured to Portsmouth City Council's design guidance, delivered in days and revised free until your decision, with a free approval-odds check before you commit.
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 homes come in a handful of recurring shapes, and each one points to a different extension. The borough is built largely from interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing, so a Victorian terrace usually wants a rear or side-return infill, while an interwar semi has room for a wider single-storey wrap or a two-storey side addition. Portsmouth City Council judges every one of them against the Portsmouth Local Plan and the council's Houses in Multiple Occupation SPD, weighing footprint, height and the impact on the house next door before it looks at the elevations.
That is why the drawings decide the outcome. We set the extension depth against the permitted-development limits precisely, keep the eaves and ridge within the allowances, and plot the 45-degree daylight lines to the neighbours' windows so the case officer can see the amenity test is passed. 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 design around it from the first sketch rather than discover it at validation.
Cosham also protects whole 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. On designated land the larger-home extension route and most side extensions stop being permitted development, so we confirm your address against those layers before drawing a line.
Plenty of Cosham extensions are permitted development and need no planning application — a single-storey rear extension within the depth and height limits is the common case. You will normally need to apply to Portsmouth City Council if any of these apply:
We draw the footprint, the ridge and eaves heights and the neighbours' daylight on the page — the relationships Portsmouth City Council's officers weigh first — and submit a validation-complete pack, whether that is a lawful development certificate, a prior-approval notification 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 complete, policy-literate extension pack anticipates those pressure points before submission, which is why professionally drawn applications are decided faster and refused less often than DIY ones.
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 extension that lands most often is the single-storey rear addition — a glazed kitchen-diner across the back of the house, kept within the permitted-development depth or taken to a full application where the ground floor wants more. On the borough's interwar semi-detached houses, a side-return infill frequently pairs with it to square off the plan; on wider postwar and modern estate housing, a two-storey side or rear extension can add a bedroom above without swallowing the garden.
The historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. We size the extension to the amenity and daylight tests, match the roof and materials to the host house, and keep enough garden to satisfy Portsmouth City Council's space standards — so the scheme answers the borough's questions before an objection can raise them. Where permitted-development rights are used up or removed, we prepare the householder application to the council's extension design guidance instead.
We draw extensions on every street in the borough. A few of the neighbourhoods where Cosham projects come up most often:
Extension drawings for Old Wymering's the historic village core around the medieval manor and church — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
Extension drawings for Hilsea Lines's the 19th-century ramparts and moat guarding the island's neck — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
Extension drawings for Cosham High Street's the mainland town centre fringed by interwar housing — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
Extension drawings for Highbury's the planned 1930s estate of semis below Portsdown Hill — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
Extension drawings for Drayton's interwar and postwar suburban housing to the east — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
Extension drawings for Portsdown fringe's houses climbing toward the chalk ridge and its forts — footprint-checked, daylight-tested and drawn to Portsmouth City Council's guidance.
The right extension in Cosham depends on your house type and how much garden you can give up. This is how the main options compare on planning route and what they add.
| Extension type | Permitted development? | What it adds | What Portsmouth City Council checks |
|---|---|---|---|
| Single-storey rear | Often, within 3m/4m depth (6m/8m via prior approval) | Bigger kitchen-diner across the back | Depth, height, neighbour daylight (45°) |
| Side-return infill | Sometimes, if within width and height limits | Squares off an L-shaped ground floor | Width vs the original house, glazing to boundary |
| Two-storey rear/side | No — full application | Extra bedroom or bathroom above | Bulk, overlooking, 7m rear-boundary gap |
| Wraparound (rear + side) | No — full application | Whole-width open-plan ground floor | Combined footprint and garden retained |
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 extension applications — most drawings are submission-ready within seven days of survey.
Tell us your Cosham postcode and house type. We confirm your conservation and Article 4 status, how much permitted-development depth you have left, and score your odds against live Portsmouth City Council decisions.
We measure the existing footprint, floor and eaves levels, boundary distances and the neighbours' window positions — the numbers the depth limits and daylight tests depend on.
Chartered designers develop the single-storey, two-storey or side extension to your brief, drawn inside the permitted-development limits or to Portsmouth City Council's extension design guidance.
We submit the certificate, prior-approval notice or application, manage validation and case-officer queries, and revise the drawings free until Portsmouth City Council issues its decision.
Not always. A single-storey rear extension up to 3 metres deep on a terrace or semi, or 4 metres on a detached house, and no higher than 4 metres, is usually permitted development. In Cosham you will need to apply to Portsmouth City Council for two-storey extensions, side extensions wider than half the house, anything forward of the front wall, deeper rear extensions under the larger-home prior-approval route, and any extension in a conservation area such as Old Wymering and Hilsea Lines.
Standard permitted development allows 3 metres on a terraced or semi-detached house and 4 metres on a detached one. The larger-home extension scheme can double that to 6 or 8 metres, but it needs a prior-approval notification to Portsmouth City Council and a neighbour-consultation period, and it does not apply on designated land such as Cosham's conservation areas. We confirm which route your property qualifies for before drawing.
Two-storey and double-storey extensions almost always need a full householder application to Portsmouth City Council, because they fall outside permitted development on most homes. The council looks closely at bulk, overlooking of neighbours and the 7-metre gap a two-storey rear extension must keep from the rear boundary. We design to those tests and show the daylight and privacy angles on the drawings. This applies to your Cosham address specifically.
A householder planning application in Cosham runs to a statutory eight-week target, a larger-home prior-approval notification to around six weeks including the neighbour-consultation period, and a lawful development certificate to eight weeks. Incomplete submissions stall at validation, so we submit a complete pack and manage officer queries to keep the decision on schedule. Portsmouth City Council is the decision-maker.
Yes — planning permission and building regulations are separate consents, and almost every extension needs both. Planning governs whether you can build it; building control governs how it is built, covering structure, insulation, drainage and fire safety. We prepare the building-regulations drawings alongside the planning set so Portsmouth City Council's building control or an approved inspector can sign the work off. This applies to your Cosham address specifically.
Every house and brief is different, so we price each Cosham extension individually and send a fixed, no-obligation quote back within one working day. It covers the full drawing pack with free revisions until Portsmouth City Council approves, and we tell you upfront whether your extension qualifies as permitted development. Call 020 7864 7245 to get started.
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Same fixed quotes, same fast drawings — wherever your project is.