Two-storey extension drawings for Cosham homes — bedroom above, living space below, drawn against the 3-metre projection and 7-metre boundary rules and to Portsmouth City Council's design guidance, 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.
Adding at first floor changes the question the case officer asks. A single-storey extension is judged mostly on daylight and garden loss; a two-storey extension is judged on overlooking, bulk against the neighbouring roofline, and whether it reads as part of the house or bolted onto it. Cosham is largely interwar semi-detached houses, historic Wymering village cottages and postwar and modern estate housing, and hipped roofs on the interwar semis and pitched tile roofs on the older cottages set what a matching roof actually means here.
The permitted-development route exists but is narrow: 3 metres of rear projection, a 7-metre stand-off from the rear boundary, eaves and ridge no higher than the original, and side-facing upper windows obscure-glazed and non-opening below 1.7 metres above floor level. Most two-storey schemes fail one of those, usually the 7-metre boundary rule on a shorter garden, and go in as a full application instead. We establish which route applies before designing, because it changes the drawing set.
The interwar semis are read as matched pairs, so hip-to-gable and roof changes are judged against the symmetry of the neighbour. 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.
Some two-storey rear extensions are permitted development. You will need a full application to Portsmouth City Council if any of these apply:
We set the projection and boundary distance on the site plan, prove the roof-pitch match on the elevations, and specify obscure glazing on the side windows — the three things Portsmouth City Council checks first — then submit a validation-complete pack.
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. Two-storey schemes attract more neighbour comment than any other householder work, so the overlooking and daylight case has to be made on the drawings.
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 schemes that succeed set the first floor back from the ground floor, keep the ridge below the original, and treat the flank wall as blank or obscure-glazed. On interwar semi-detached houses, matching the eaves line matters more than matching the brick: an extension that carries the existing eaves through reads as original even in a different material.
The historic Wymering village core and the scheduled Hilsea Lines set heritage limits on the mainland. We draw the 45-degree daylight test from the neighbour's nearest habitable window, show the window positions in section, and set out the steel and foundation implications early — a two-storey extension changes the load path, so the structural design and the planning design have to be developed together rather than in sequence.
We draw two-storey extensions across Cosham. A few of the areas where they come up most often:
Two storey extension drawings for Old Wymering — the historic village core around the medieval manor and church — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
Two storey extension drawings for Hilsea Lines — the 19th-century ramparts and moat guarding the island's neck — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
Two storey extension drawings for Cosham High Street — the mainland town centre fringed by interwar housing — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
Two storey extension drawings for Highbury — the planned 1930s estate of semis below Portsdown Hill — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
Two storey extension drawings for Drayton — interwar and postwar suburban housing to the east — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
Two storey extension drawings for Portsdown fringe — houses climbing toward the chalk ridge and its forts — dimensioned against the 3m and 7m limits and drawn to Portsmouth City Council's guidance.
The first-floor limits are tighter than the ground-floor ones. This is how the common cases compare.
| Scheme | Permitted development? | Deciding limit | What Portsmouth City Council checks |
|---|---|---|---|
| Two-storey rear, 3m projection, 7m+ to boundary | Often yes | 3m projection / 7m boundary | Roof match, side-window glazing |
| Two-storey rear, under 7m to boundary | No — full application | Rear-boundary distance | Overlooking, daylight, garden loss |
| Two-storey side extension | No — full application | Side elevation | Gap to boundary, street rhythm |
| Two-storey wraparound | No — full application | Combined footprint | Bulk, massing, neighbour amenity |
| Any two-storey on designated land | No — full application | Designation | Character and appearance |
Limits from the GPDO 2015, Schedule 2, Part 1, Class A — two-storey rear extension conditions.
A fixed process built around how Portsmouth City Council validates and decides two-storey applications.
Send us your Cosham postcode and garden depth. We measure the 7-metre boundary rule against your plot, confirm designation status, and score your odds against live Portsmouth City Council decisions.
We measure the original rear wall, eaves and ridge heights, garden depth and every neighbouring window that faces the site — the inputs the daylight and overlooking case is built from.
Chartered designers develop the two-storey addition to your brief with the roof and eaves resolved, and we coordinate the load path so the structural scheme and the planning scheme are the same building.
We submit the application, manage validation and case-officer queries, and revise the drawings free until Portsmouth City Council issues its decision.
Often, but not always. A two-storey rear extension can be permitted development if it projects no more than 3 metres, sits at least 7 metres from the rear boundary, does not exceed the existing eaves and ridge, and has obscure-glazed non-opening side windows below 1.7 metres. In Cosham, the 7-metre rule is what most commonly forces a full application to Portsmouth City Council.
No part of a permitted-development two-storey rear extension may come within 7 metres of the boundary opposite the rear wall of the house. It exists to protect the neighbour behind you from being overlooked at first floor. On shorter Cosham gardens this is decisive, and we measure it on survey before any design work rather than discovering it at validation. Portsmouth City Council is the decision-maker.
Not under permitted development — two-storey side extensions always need a full application to Portsmouth City Council. They are assessed on the gap to the boundary, whether the street's rhythm of spaces between houses survives, and whether the extension is subordinate to the original. Setting the first floor back from the front building line and down from the ridge is what usually makes the difference. This applies to your Cosham address specifically.
Overlooking is the most common objection, and it is answerable on the drawings. We position habitable-room windows away from neighbouring gardens, obscure-glaze and fix shut anything on the flank below 1.7 metres, and show the sight lines in section. Where the geometry is tight, a rooflight or a high-level window gives the same daylight without the mutual overlooking that draws objections. This applies to your Cosham address specifically. Portsmouth City Council is the decision-maker.
Almost certainly. Excavating within 3 metres of a neighbouring structure and building on or up to the boundary both trigger the Party Wall etc. Act 1996, and a two-storey extension usually does both. It is entirely separate from Portsmouth City Council's planning decision, so we flag the notice period early and detail the junction on the drawings. This applies to your Cosham address specifically.
Yes. Adding a floor changes the load path from roof to foundation, so beams, padstones and foundation depths all need designing and calculating for building control. Planning and building regulations are separate consents, and the structural set is what building control assesses — we prepare both so the approved scheme is the buildable one. This applies to your Cosham address specifically. Portsmouth City Council is the decision-maker.
We quote each Cosham two-storey extension individually, because plot depth, roof form and structural complexity vary house to house, and send a fixed, no-obligation quote within one working day. It covers the drawing pack with free revisions until Portsmouth City Council approves. 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.