Modern Flats in London: Planning Permission Guide

The modern London flat: defining characteristics

“Modern flat” in this guide covers anything from a 1960s LCC-era block through to a 2024 regeneration scheme in Nine Elms or Stratford. London flats share certain technical and legal features regardless of era:

• Leasehold ownership — you own a long lease (typically 125 or 999 years) of the interior, not the structural walls, roof, foundations or external envelope
• Freehold sits with a landlord — sometimes a housing association, sometimes a property company, sometimes a Residents’ Management Company (RMC) where leaseholders jointly own
• Compartmentation — each flat is a fire compartment; floors and walls must resist fire spread for specific periods (typically 60 minutes)
• Party structures above and below — the floor slab is a party structure under the Party Wall etc. Act 1996
• Concrete or timber floor construction — older blocks (1960s–80s) have reinforced concrete floors; some newer builds use CLT or engineered timber
• Shared services — soil stacks, cold and hot water risers, extract ducts all run between flats
• Balconies or Juliette balconies — usually demised to the flat but with restrictive covenants on enclosure or external changes
• Communal spaces — lobbies, corridors, stair cores are the landlord’s responsibility and cannot usually be altered by leaseholders

Planning considerations: the permission stack

Because flats sit under a more restrictive regime than houses, you typically need three separate permissions before starting any significant work:

1. Planning permission from the local authority — needed for virtually any alteration visible externally (new windows, solar panels, balcony enclosure, change of use), any change of layout that affects parking or amenity, and any works to listed or conservation-area buildings
2. Leaseholder consent — almost every lease has covenants requiring you to obtain the landlord’s written consent before structural alterations, changes to services, or installation of hard flooring. Consent is typically chargeable (£500–3,000) and may require a professional’s sign-off
3. Freeholder / management company approval — even where the landlord is just a property management firm, they must approve and often require indemnity insurance, scheduled works notices and a license to alter

Critically, Permitted Development is almost entirely unavailable for flats. The General Permitted Development Order 2015 strips most PD rights from flats and maisonettes; what’s left is limited to things like internal decoration that wouldn’t need permission anyway.

Read our full planning permission guide for the general process, but for a flat project we strongly recommend a pre-application enquiry and a specialist leasehold-friendly design approach.

Typical projects on a modern flat

Internal layout reconfiguration

The single most common flat project. Knocking through a wall between kitchen and living room, moving a bathroom, creating an en-suite, or converting a study into a second bedroom. Even though it’s internal:

• Leaseholder consent is almost certainly required if the wall is structural or shared
• Freeholder approval and licence to alter typically mandatory
• Building Regulations apply for any structural change, wet-room, or new kitchen
• Part E (resistance to sound passage) applies if you’re changing the demising floor, ceiling or separating walls
• Fire compartmentation must be maintained — any penetration for services needs re-sealing to FD60 or equivalent

New bathroom / wet-room

Moving or adding a bathroom is surprisingly complicated in a flat. Soil stacks run vertically through the building, so a new WC must connect within 6m of an existing stack (with fall). Waterproof tanking, macerator pumps (rarely loved by neighbours), and noise attenuation on the pump are all required. Expect to supply the freeholder with a method statement for the works.

Solid floor finishes (wood, tile, stone)

Many flat leases prohibit hard flooring in living areas. Where permitted, you’ll need to meet the lease’s minimum acoustic rating (often 65–68dB L′nT,w). That typically means an acoustic underlay of 20–30mm build-up — a significant floor-to-ceiling height hit in low-head flats. A Part E test may be commissioned at your cost.

External window replacement

Usually requires planning permission (even where houses wouldn’t, because the flat sits within a larger whole building). Replacement must typically match the original in material, colour and profile. Lease will require matching freeholder spec.

Balcony enclosure / glazed balcony

Almost always requires planning. Many London boroughs refuse balcony enclosures on amenity grounds. Some modern developments have standing planning conditions that prohibit enclosure.

Loft conversion (top-floor flat)

If you own the top-floor flat, you do not automatically own the roof space — it’s usually demised to the freeholder. Acquiring airspace rights is a separate lease negotiation (often £30,000–80,000 in inner London) before any loft project can begin. Then you still need planning.

Typical projects compared

Structural and regulatory quirks

Flats add a dense layer of rules and technical constraints on top of what a house project would need:

• Fire compartmentation (Building Regs Part B) — each flat is an individual compartment. Every penetration through a demising wall or floor must be properly fire-stopped. Post-Grenfell, compartmentation has become the single most inspected item
• Part E (resistance to sound) — separating walls and floors must achieve specific airborne and impact sound ratings. Any alteration that reduces acoustic performance is a breach of Regs
• Building Safety Act 2022 — flats in buildings 18m+ or 7+ storeys are subject to the Building Safety Regulator, with a gateway approval process adding significant time and cost
• Loadbearing walls — in concrete-frame blocks, internal partitions are usually non-structural, which is helpful. In cross-wall construction (common in 1960s LCC blocks), all the cross walls are structural
• Services risers — soil stacks, hot/cold water, heating flow/return, gas, fibre — all run through shared risers and can’t be rerouted without consent
• Ceiling heights — often tight (2.3–2.5m). Any floor build-up for acoustic performance eats into head height
• Heat pumps and AC — external condensers almost always need planning permission and are often refused in conservation areas. Heat-recovery systems inside the flat are easier

Our building regulations drawings service for flats always includes Part E acoustic specification, Part B fire compartmentation detail, and a coordination check with any existing M&E risers.

Party Wall: three potential neighbours, not two

This is the Party Wall Act scenario flat owners most often miss. On a flat:

• Side neighbours — notice under Section 2 for any wall-penetrating work
• The flat above you — your ceiling is their floor. Notice under Section 2 if you’re affecting the ceiling/floor structure, and Section 6 if you’re excavating (rare in flats but applies to lightwell or basement flats)
• The flat below you — your floor is their ceiling. Same situation in reverse

All three relevant neighbours must be served notice 2 months before work starts (1 month for Section 6). In a building with a Residents’ Management Company, the RMC may also require notice on top of the Party Wall process.

Costs: modern flat projects in 2026

Flat projects tend to have lower construction cost per sqm than houses (less square footage, no external envelope work) but higher fees and consents — typically adding 8–15% on top of the build. See our fixed-fee pricing.

Services we provide for flats

Our experience with London flats

Flat projects account for roughly 10% of our portfolio, but they’re disproportionately the tricky ones — not because the drawings are complex, but because the consent stack is. We routinely coordinate planning, leaseholder licence-to-alter, Party Wall, Building Safety Regulator (where 18m+) and conservation-area considerations on a single project. Our drawings pack for a flat alteration typically includes: existing plans, proposed plans, demolition plan, structural annotations, fire compartmentation schedule, Part E sound schedule, and a method statement addressing services.

The mistake we see most often: owners starting work on the basis of planning permission alone, without leaseholder consent in place. If the freeholder objects later, you may be forced to reinstate at your own cost. Get every consent signed before any demolition. Get a fixed-fee quote.

Flat services by London borough

Flats dominate the inner-London housing stock; we work across all 33 boroughs:

Frequently asked questions