Extending a flat at ground floor into a garden

A ground floor flat extension needs planning permission every time. Flats have no permitted development rights at all, because Part 1 of the General Permitted Development Order applies to dwellinghouses and expressly excludes a flat and any building containing one. So the small rear infill your neighbour in the terraced house built without an application is not available to you, and the gap between those two positions is where most of the frustration in this subject comes from.

There is a second difference that costs people money: this is not a householder application.

No permitted development rights, and a different form

The householder application route is for alterations and extensions to a single dwellinghouse. A flat does not qualify, so an extension to a ground floor flat goes in as a full application for minor development, on the full form, with the fee set out in the current fee regulations.

That has three consequences worth knowing before you start.

The description of development and the certificates are different, and the ownership certificate usually has to name the freeholder and any other leaseholders, with notice served on them.

The publicity is the standard 21 day consultation, and in practice the other leaseholders in the building will be notified.

And the appeal route changes. The expedited householder appeal service with its 12 week timetable is not available for a flat, so a refusal goes to a standard written representations appeal with the longer deadline and the longer timetable. By contrast, rear dormers on a semi-detached house may not need an application at all, which tells you how differently the two are treated.

Policy on extensions to flats

Local plans rarely have a policy headed "extensions to flats". The requirement is assembled from the general residential amenity policy, the design policy, and any supplementary design guide.

Officers assess four things.

Whether the extension harms the amenity of the other flats in the building, which is the point of difference from a house. In a house the amenity at stake belongs to the neighbours alone. In a converted building, the flats above are affected by what happens at ground level.

Whether enough usable outdoor amenity space remains for the building as a whole, or for the flat where the garden is demised to it.

Whether the extension reads as a subordinate addition or as an over-development of the plot, measured against the rhythm of the rear elevations along the terrace.

Whether a precedent is created that the authority would struggle to resist on the neighbouring buildings.

That last point is real rather than rhetorical. On a terrace of converted houses, approving one full-width rear extension at ground floor makes the next eight harder to refuse, and officers price that into their recommendation.

Amenity space, and who owns the garden

Establish the tenure of the garden before you measure anything. Three situations arise.

The garden is demised to your flat in the lease. You have the strongest position, and the amenity space question is about how much of your own space you are willing to lose.

The garden is communal, held by the freeholder for the benefit of all flats. Extending into it needs the freeholder's agreement and, in practice, the other leaseholders' acceptance, and the planning case has to explain how the remaining communal space still works.

The garden is demised but subject to rights of access for the other flats, commonly for bin storage, cycle parking or a fire escape route. Those rights have to survive the extension, and if the sole bin store disappears the application will be refused on amenity grounds whatever the lease says.

Where a local standard sets a minimum private outdoor space per flat, measure what is left against it and put the figure on the drawing. In London the plan sets a baseline for private outdoor space with minimum dimensions, which means a 1.2 metre strip left behind the extension will not count.

Overlooking and the flat roof problem

The extension roof generates more conditions than anything else. A flat roof at ground floor sits immediately below the upstairs windows of the flat above, and two things follow.

The occupiers above can see down onto it, and onto whatever is behind it. That is usually acceptable, since they could already see the garden.

The bigger issue is that the roof could become a terrace. Councils routinely impose a condition that the flat roof shall not be used as a balcony, roof garden or sitting out area, and they mean it. Some go further and require the roof to be finished in a material unsuitable for occupation, or require a guard that prevents access.

Design it out rather than arguing. A pitched or mono-pitch roof removes the question. A flat roof with no door or window giving access to it from above does the same. Where the upper flat has a window overlooking a new terrace area, expect the kind of control described in obscure glazing conditions on side facing windows, applied to the rear instead.

Daylight to the flats above

A full-width rear extension projecting 3 or 4 metres sits directly under the upstairs windows, and on a north-facing rear elevation that matters.

Run the 45 degree test from the centre of the closest first floor window in plan and in section, and if the extension crosses the line, commission a proper assessment. A Daylight and Sunlight Assessment against the BRE guidance costs a fraction of a refused application, and it also tells you whether reducing the depth by half a metre fixes the problem.

Depth and width together decide this. A 4 metre deep extension across the full width is far more damaging than a 4 metre deep extension across two thirds, with a set-back allowing light down the side.

Lease, freehold and party wall consents

None of these are planning matters, and all of them determine whether the project happens.

Your lease almost certainly prohibits structural alterations without the landlord's written consent. Where the lease says consent is not to be unreasonably withheld, statute supports you to a degree, but the process still takes time and the landlord can charge their reasonable costs.

If the extension occupies land outside your demise, a licence is not enough. You need a deed of variation extending the demise, or a transfer of the relevant land, and the lender on every flat in the building may need to be told.

Where the works affect a party wall or involve excavation near a neighbouring structure, the Party Wall etc. Act 1996 applies on its own timetable, independent of planning.

Start the lease conversation before the planning application rather than after. A permission you cannot implement is an expensive piece of paper.

Drainage and shared services

Ground floor flat extensions sit on top of the drainage for the whole building. Expect to find a soil stack from the upper flats discharging into a gully or inspection chamber exactly where you want the new kitchen.

Deal with it on the drawings. Show the existing runs, the proposed diversions, and maintenance access. Where the extension would be built over or near a public sewer, the sewerage undertaker's build over agreement is a separate consent with its own lead time, and the Building Regulations deal with the construction detail.

Also check the gas and electricity supply boxes, the water stopcock and any shared rainwater goods. Burying another flat's supply box inside your extension is a problem you will be made to solve later.

Evidence that gets it approved

• A site plan showing the garden ownership and the remaining amenity space with areas stated.
• Existing and proposed plans and sections, with the 45 degree line drawn to the upstairs windows.
• A roof design that removes the terrace question, or a stated acceptance of the no-use condition.
• A drainage layout showing existing runs, diversions and access.
• A short Planning Statement addressing the amenity of the other flats specifically, not just the neighbours.
• Confirmation that the freeholder and other leaseholders have been notified, with the certificates completed correctly.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the formal way to confirm whether works are lawful.

Enter your postcode on PlanScope to see which supporting documents a flat extension is likely to need.

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