Extending a ground floor flat

A ground floor flat with its own garden is one of the best extension opportunities in London, and one of the least attempted, because the process has an extra layer that houses do not — and most general advice online quietly assumes you own a house.

Everything below applies to a flat or maisonette. The building work itself is not fundamentally different from extending a house. The permissions are.

There are no permitted development rights. None.

Householder permitted development applies to houses. Flats and maisonettes are excluded from all of it — rear extensions, side extensions, roof extensions, outbuildings, the lot.

So the first thing to accept is that any extension to a flat requires a full planning application, however small, however invisible from the street. There is no three-metre allowance and no lawful development certificate route. Householder application, eight weeks from validation, decided against the local plan.

The upside of this is that it removes an entire category of ambiguity. You are not measuring eaves against boundaries hoping to squeak inside a rule. You are designing the right scheme and applying for it.

Your lease decides more than your council does

Planning permission grants you a planning right. It does not grant you a property right. Those are separate, and on a leasehold flat the second one is the harder of the two.

What to check, in order:

• What is actually demised to you. Read the parcels clause and the plan. Many flat leases demise the internal surfaces only — the external walls, the structure, the roof and often the garden belong to the freeholder even where you have exclusive use of the garden. If the garden is not demised, extending into it is not simply your decision.
• The alterations covenant. Most leases prohibit structural alterations without the landlord's consent, frequently with an express requirement for consent in writing. Some are absolute prohibitions; many are qualified, meaning consent may not be unreasonably withheld.
• Whether a licence to alter is required. This is the formal document that gives consent, usually with conditions attached about specification, insurance, timescale and reinstatement. Expect to pay the freeholder's legal and surveying costs.
• Whether the demise needs varying. If the extension occupies land outside your demise, consent alone may not be enough — you may need a deed of variation to bring the new footprint into the lease. Skipping this stores up a serious problem for the day you sell.

If the building is share of freehold, the same requirements still apply — you cannot consent to yourself informally. The consent needs documenting between the company and you as leaseholder, and the other leaseholders need to agree it properly. "The neighbours are fine with it" is not a document a buyer's solicitor will accept in ten years' time.

Also tell your mortgage lender and your buildings insurer. Both usually want to know, and both are far easier to deal with in advance.

The flat above changes the structure

In a house, the walls you are opening up carry your own floors. In a converted flat they carry somebody else's.

That has three practical consequences. Loads have to be transferred through the new structure with the upper flat's loading properly accounted for — this is engineer's work, not rule-of-thumb work. Temporary support during the opening-up has to be designed rather than improvised, because the propping is holding up an occupied home. And the sequence of work matters more, because the people above cannot simply move out for a fortnight.

The floor between you and the flat above is, for the purposes of the Party Wall etc. Act, a party structure. Cutting into it or into the walls that support it is notifiable, and your upstairs neighbour is an adjoining owner with the same rights as the person next door — as is the freeholder. Party wall notices on a converted building routinely go to three or four people.

Fire and sound do not get a concession

Building control on a flat is stricter than on a house, because the building regulations treat a block of flats as a set of separate fire compartments. The separation between your flat and the one above has to be maintained through everything you build. New openings, new services running through the compartment line, a new ceiling — all of it has to preserve the fire resistance and the sound insulation that was there before.

You also need to keep a viable escape route from the extended space, and the extension must not compromise anyone else's. On the ground floor flat we worked on in Herne Hill, the escape provision for the flats above ran through the shared courtyard — which meant the extension had to be designed around keeping that route clear, and that constraint shaped the whole scheme rather than being a detail at the end.

The order to do it in

The sequence that avoids wasted money:

• Read the lease, or have a solicitor read it, before you commission any drawings. If the alterations covenant is absolute and the freeholder is hostile, you want to know that on day one.
• Approach the freeholder informally with a sketch scheme. Their view shapes the design.
• Survey, structural principle, drawings.
• Planning application and licence to alter running in parallel — they are independent, and running them in sequence adds months for no reason.
• Building control, party wall notices, then build.

Done in that order, extending a flat takes longer on paper than extending a house and roughly the same amount of time on site. Done in the wrong order, it takes a year longer and costs a set of drawings.