Side-return infill drawings for Cosham terraces — the glazed extension that fills the redundant side alley and opens up the kitchen, drawn to Portsmouth City Council's guidance, 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 side return is the classic terrace fix, and Cosham has the stock for it — the borough is built largely from interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing, and the Victorian and Edwardian versions almost all have an L-shaped back addition with a wasted alley beside it. Fill that alley and the dark galley kitchen becomes a bright, square room the full width of the house. Portsmouth City Council judges the infill against the Portsmouth Local Plan and the council's Houses in Multiple Occupation SPD, and because it runs along the party boundary, the neighbour relationship is read before the design.
That boundary is where the drawings earn their keep. We set the infill wall on or up to the line, keep the roof — usually a glazed lantern or a shallow pitch tying into the existing back-addition slope — below the neighbour's first-floor windows, and detail the rainwater and party-wall junction so the build is watertight and the Party Wall Act is satisfied. 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 section rather than leave it to the case officer.
Many of Cosham's terraces sit in conservation areas — Old Wymering and Hilsea Lines among them — where a side extension is no longer permitted development. 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 a line is drawn.
Some single-storey side returns in Cosham are permitted development, but the width and boundary make an application more likely than for a straight rear extension. You will normally need to apply to Portsmouth City Council if any of these apply:
We draw the infill wall on the boundary, the glazed roof below the neighbour's windows and the party-wall junction on the page — the details Portsmouth City Council's officers and your neighbour both check — 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. Side returns hinge on width, roof glazing and the party boundary, so a pack that resolves those details upfront is decided 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 approved side return is a single-storey infill roofed with a glazed lantern or roof light to pull daylight deep into the plan, its flank wall built on the boundary and finished to match the host brick. On the borough's interwar semi-detached houses, this is frequently the single biggest gain in usable ground-floor space, and where it stays within the width and height limits it can proceed under permitted development or a lawful development certificate.
The historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. We keep the glazed roof below the neighbour's cill line, set the gutter and the party-wall flashing so water and maintenance are designed in, and — where owners want the full open-plan room — pair the infill with a rear extension as a single wraparound application. That way Portsmouth City Council sees one coherent scheme rather than two overlapping ones.
We draw side-return infills on every terraced street in the borough. A few of the neighbourhoods where Cosham side returns come up most often:
Side return extension drawings for Old Wymering's the historic village core around the medieval manor and church — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Side return extension drawings for Hilsea Lines's the 19th-century ramparts and moat guarding the island's neck — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Side return extension drawings for Cosham High Street's the mainland town centre fringed by interwar housing — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Side return extension drawings for Highbury's the planned 1930s estate of semis below Portsdown Hill — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Side return extension drawings for Drayton's interwar and postwar suburban housing to the east — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Side return extension drawings for Portsdown fringe's houses climbing toward the chalk ridge and its forts — width-checked, party-wall detailed and drawn to Portsmouth City Council's guidance.
Whether a side return in Cosham needs permission comes down to width, storeys and whether the street is designated. This is how the cases compare.
| Side return type | Permitted development? | What it gains | What Portsmouth City Council checks |
|---|---|---|---|
| Single-storey infill, within limits | Often yes, if under half the house width | Full-width kitchen-diner | Width, 4m height, roof glazing |
| Single-storey, over half width | No — full application | Wider room, more light | Boundary treatment, neighbour amenity |
| Two-storey side return | No — full application | Room above as well as below | Bulk, overlooking, terrace rhythm |
| Wraparound (side + rear) | No — full application | L-shaped open-plan ground floor | Combined footprint, garden retained |
In conservation areas such as Old Wymering and Hilsea Lines, side extensions are not permitted development and always need a full application.
A fixed process built around how Portsmouth City Council validates and decides side-return applications — most drawings are submission-ready within seven days of survey.
Send us your Cosham postcode and terrace type. We confirm your conservation and Article 4 status, whether the infill stays within the side-extension width limit, and score your odds against live Portsmouth City Council decisions.
We measure the side alley, the back-addition wall, floor and eaves levels and the neighbour's window heights — the numbers the width limit, roof pitch and party-wall detailing depend on.
Chartered designers develop the infill and its glazed roof to your brief, drawn inside the permitted-development limits or to Portsmouth City Council's extension guidance, with the boundary and party-wall junction detailed.
We submit the certificate or application, manage validation and case-officer queries, and revise the drawings free until Portsmouth City Council issues its decision.
Sometimes not. A single-storey side-return infill can be permitted development if it is no wider than half the original house, no higher than 4 metres, and the property is not on designated land. In Cosham you will need a full application to Portsmouth City Council for anything wider or taller, for a two-storey side return, and for any side extension in a conservation area such as Old Wymering and Hilsea Lines, where those rights are removed.
It is an infill that fills the narrow open alley running alongside the rear back-addition of a Victorian or Edwardian terrace — the strip of ground that is usually too small to use. Building over it squares off the ground floor, so a cramped galley kitchen becomes a full-width room. Cosham's period terraces are ideally suited to it, which is why the side return is one of the borough's most common extensions. Portsmouth City Council is the decision-maker.
Almost always. A side return is built on or up to the boundary with your neighbour, so the Party Wall etc. Act applies and you must serve notice before work starts — this is entirely separate from Portsmouth City Council's planning decision. We detail the party-wall junction on the drawings and flag the notice early so a surveyor can be appointed without delaying the build. This applies to your Cosham address specifically.
Yes, and it is common — the combined L-shape is a wraparound extension. Once the side infill joins a rear projection beyond the permitted-development depth, the whole thing needs a full householder application to Portsmouth City Council rather than qualifying as permitted development. We draw it as one coherent scheme so the council assesses a single footprint, which reads more clearly than two overlapping proposals. This applies to your Cosham address specifically.
A householder planning application in Cosham runs to a statutory eight-week target, and a lawful development certificate to eight weeks as well, though an incomplete pack can stall at validation. Because a side return involves the boundary and roof glazing, a complete, well-detailed submission matters — we prepare it validation-complete and manage officer queries to keep the decision on track. Portsmouth City Council is the decision-maker.
We price each Cosham side return individually, because alley width and roof form 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 infill is permitted development or needs an application. Call 020 7864 7245 to start.
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