Annexe and granny-flat drawings for Cosham homes — designed to stay ancillary to the main house rather than become a separate dwelling, drawn to Portsmouth City Council's policies, delivered in days and revised free until your decision.
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.
Councils do not decide annexes on square metres. They decide whether, once built, the accommodation forms part of one planning unit with the main house or a second one beside it. Portsmouth City Council looks at whether the garden is shared or divided, whether the annexe has its own street access and address, whether the occupier could live there entirely independently, and whether the two households would function as one. That is the whole case, and it is made or lost on the layout.
Cosham is largely interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing, which shapes the realistic options: converting part of the existing house, extending it, or building a detached unit in the garden. A detached annexe can sometimes sit under Class E as an outbuilding incidental to the enjoyment of the dwellinghouse — but sleeping accommodation is generally not "incidental", so a garden annexe with a bedroom usually needs its own permission rather than relying on outbuilding rights.
In Cosham's conservation areas — including Old Wymering and Hilsea Lines — a garden building is also judged on its effect on the character of the area, which limits height and footprint before the use question is reached. 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.
An annexe almost always needs an application to Portsmouth City Council. Expect one where any of these apply:
We plan the annexe so its relationship to the main house is visible on the page — shared garden, connected access, a layout that reads as one household — and set out the case Portsmouth City Council has to test, rather than leaving the officer to assume the worst.
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. Annexe applications are decided on use as much as design, so the supporting case matters as much as the elevations.
Source: MHCLG planning application statistics (householder decisions, England, 1 April 2025 - 31 March 2026). Compare every borough on our London approval-rate league table.
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Across Old Wymering, Hilsea Lines and Cosham High Street, the annexes that succeed keep one shared garden, take access past the main house rather than from the street, and are modest against the host building — single storey where the plot is overlooked, and set back from boundaries. On interwar semi-detached houses, converting an integral garage or a rear ground floor is often approved more readily than a detached garden unit, because the ancillary relationship is self-evident.
The interwar semis are read as matched pairs, so hip-to-gable and roof changes are judged against the symmetry of the neighbour. Where Portsmouth City Council grants permission, it commonly attaches a condition tying occupation of the annexe to the main dwelling. That condition is not a problem to be avoided — it is usually the mechanism that makes approval possible, and we draft the application on that basis rather than pretending the annexe is something it is not.
We draw annexes across Cosham. A few of the areas where multigenerational schemes come up most often:
Annexe and granny-flat drawings for Old Wymering — the historic village core around the medieval manor and church — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Annexe and granny-flat drawings for Hilsea Lines — the 19th-century ramparts and moat guarding the island's neck — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Annexe and granny-flat drawings for Cosham High Street — the mainland town centre fringed by interwar housing — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Annexe and granny-flat drawings for Highbury — the planned 1930s estate of semis below Portsdown Hill — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Annexe and granny-flat drawings for Drayton — interwar and postwar suburban housing to the east — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Annexe and granny-flat drawings for Portsdown fringe — houses climbing toward the chalk ridge and its forts — planned to stay ancillary and drawn to Portsmouth City Council's policies.
Three similar buildings, three different consents. This is what separates them.
| What you build | Planning status | Key test | What Portsmouth City Council checks |
|---|---|---|---|
| Garden room, no sleeping space | Often Class E permitted development | Incidental to the house | Height, footprint, boundary position |
| Annexe used by the household | Full application, usually with a tie condition | Ancillary use | Shared garden and access, layout |
| Self-contained unit, own access | New dwelling — full application | Separate planning unit | Policy on new homes, amenity, parking |
| Converted garage or ground floor | Often permitted, use still assessed | Whether use changes | Independence of the accommodation |
| Annexe on designated land | Full application | Designation | Character and appearance |
The ancillary/separate-dwelling distinction comes from section 55 of the Town and Country Planning Act 1990; outbuilding limits from the GPDO 2015, Schedule 2, Part 1, Class E.
A fixed process built around how Portsmouth City Council tests the use question as well as the design.
Tell us who the annexe is for and where it would sit on the plot. We confirm designation status and tell you honestly whether the scheme reads as ancillary or as a separate dwelling in Cosham.
We measure the plot, the existing house and the access route, so the shared-garden and shared-access case can be drawn rather than asserted.
Chartered designers develop the annexe to your brief, with the layout, access and garden relationship arranged to support the ancillary case under Portsmouth City Council's policies.
We submit the application with a supporting statement on use, manage validation and case-officer queries, and revise free until Portsmouth City Council decides.
Almost always. Sleeping accommodation is generally not "incidental" to the enjoyment of a house, so it falls outside the outbuilding permitted-development right, and any annexe that could function independently raises a change-of-use question under section 55. In Cosham we prepare a full application to Portsmouth City Council and make the ancillary case explicitly rather than hoping it is not asked.
A shared garden rather than a divided one, access through the main property rather than a separate street entrance, occupation by a member of the same household, and a physical and functional relationship with the main house. Portsmouth City Council weighs those together — no single feature decides it, but a separate front door and a fenced-off garden together usually settle it against you. This applies to your Cosham address specifically.
Yes, subject to permission. A detached garden annexe is assessed on its scale against the plot, its effect on neighbours and the character of the area, and then on the use question. Keeping it single storey, set in from boundaries and served off the shared garden gives it the best prospect in Cosham, particularly near designated areas such as Old Wymering and Hilsea Lines. Portsmouth City Council is the decision-maker.
Commonly, yes — a condition tying occupation of the annexe to the main dwelling, so it cannot be sold or let separately. That is normally what makes approval possible, because it removes the concern that a new independent home is being created without going through housing policy. The condition runs with the land, so it matters at resale, and we explain the implications before you commit. This applies to your Cosham address specifically. Portsmouth City Council is the decision-maker.
Yes. A new annexe is new habitable accommodation, so structure, fire escape, insulation and ventilation all apply, and a detached annexe close to a boundary raises fire-spread requirements as well. Planning and building control are separate consents with separate submissions — we prepare both so the approved design is the one that can actually be built. This applies to your Cosham address specifically. Portsmouth City Council is the decision-maker.
A householder application runs to an eight-week statutory target; where the council treats the annexe as creating a new unit it becomes a minor application with a thirteen-week target instead. Which route applies depends on how the use is characterised, which is precisely why the ancillary case is worth making properly at the outset. This applies to your Cosham address specifically. Portsmouth City Council is the decision-maker.
We quote each Cosham annexe individually, because plot, access and whether you are converting or building new all change the work, and send a fixed, no-obligation quote within one working day. It covers the full drawing pack and supporting statement, with free revisions until Portsmouth City Council approves. Call 020 7864 7245 to start.
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