Westminster Extension Guide 2026: Planning Permission, Conservation Rules & Costs
Westminster is the most heritage-constrained borough in London for anyone planning a house extension. A large majority of the city sits inside a conservation area, tens of thousands of buildings and structures carry listed status, and the council was among the first in the country to introduce a dedicated basement policy. Here is what you actually need to know before extending in Westminster.
TL;DR - The Quick Answer
Westminster combines near-total conservation area coverage with one of the highest concentrations of listed buildings of any local authority in England. According to the council's own figures, 56 conservation areas cover more than three-quarters of the city, and around 11,000 buildings and structures are listed. The practical effect for homeowners: most permitted development rights simply do not apply, and even a modest single-storey rear extension is likely to need full planning permission. Basements face further restriction: a borough-wide Article 4 direction has removed all permitted development rights for basement excavation since July 2016, and City Plan policy limits new basements to a single storey with no more than half the garden excavated. Build costs sit at the premium end of London pricing, typically 15-25% above the London average. Given how easily a property can fall inside a conservation area boundary or carry listed status without an owner realising it, confirming planning status formally before design work starts matters more in Westminster than almost anywhere else in the capital.
Westminster: A Uniquely Constrained Borough
The City of Westminster is small, dense, and extraordinarily historic. It contains the seat of national government, the core of Georgian and Regency London, and some of the most recognisable streetscapes in the country - the stucco terraces of Belgravia and Pimlico, the planned squares of Marylebone, the villa terraces of St John's Wood, and the grand townhouses of Mayfair. Almost every street has some form of heritage designation attached to it.
That density of protection is not incidental. Westminster council applies conservation and heritage controls more comprehensively than almost any other local authority in England, so the sensible starting assumption is the opposite of most London boroughs: assume full planning permission is required unless you can prove otherwise, rather than assuming permitted development applies. None of this makes extending in Westminster impossible - extensions are approved routinely every year - but the process demands more preparation, a higher standard of design and materials, and a realistic budget for both build costs and professional fees than in almost any outer London borough.
Conservation Areas: More Than Three-Quarters of the City
According to Westminster City Council, the city has 56 conservation areas, and the council's own published overview of these areas states that they cover more than 76% of the city - one of the highest proportions of any local authority in the country. Well-known Westminster conservation areas include Mayfair, Belgravia, Marylebone, St John's Wood, Dorset Square, the Savoy, and the area around Westminster Cathedral, alongside dozens of smaller residential conservation areas across the borough.
Being inside a conservation area does not ban extensions outright, but it removes or curtails most permitted development rights. In practice this means:
What conservation area status changes
A number of individual conservation areas go further still, with their own Article 4 directions removing rights for minor works such as painting brickwork, replacing windows, or altering front boundaries - Queen's Park Estate, Sussex Gardens, and Bristol Gardens are among them. Our guide to conservation area extensions in London covers the general rules underneath this borough-specific detail.
Listed Buildings: One of the Highest Concentrations in England
Westminster City Council states that the statutory list includes around 11,000 buildings and other structures in the city - one of the highest totals of any single local authority in England, and a figure that reflects the sheer density of Georgian, Regency, and Victorian townhouses across the borough.
If your property is listed, you need listed building consent in addition to planning permission for any extension, and often for internal works too. This is a separate consent regime, assessed against different criteria to standard planning permission, and it is a criminal offence to carry out unauthorised works to a listed building. Key points:
Listed building consent basics
Listing protects the whole building, not just the facade. Removing a wall, altering a staircase, or changing an original fireplace can all require consent, even where no external change is proposed.
Outbuildings, mews structures, boundary walls, and garden features within the grounds of a listed building can also be protected, even if not separately listed themselves.
Most permitted development rights that might otherwise apply to a house do not extend to a listed building. Almost any physical change needs consent.
Given the scale and complexity of listed building stock in Westminster, a design team with direct experience of the council's heritage officers and Historic England consultation (for Grade I and II* buildings) is strongly advisable.
Because conservation areas and listed buildings frequently overlap in Westminster, many projects need to satisfy both regimes at once - a planning application assessed against conservation area character, and a listed building consent application assessed against the significance of the building itself. Our guide to listed building extensions in London covers the general process.
What Permitted Development Survives in Westminster
Given the scale of conservation area coverage, standard national permitted development rights for house extensions only apply in earnest to the minority of Westminster properties that sit outside a conservation area, are not listed, and are not affected by a property-specific Article 4 direction. Even then, three things are worth checking before assuming anything:
| Article 4 direction | In force since | What it removes |
|---|---|---|
| Basement development (borough-wide) | 31 July 2016 | All permitted development rights for basement excavation, anywhere in the city |
| Class E / office to residential (Central Activities Zone) | 2021, modified 2022 | Change of use from commercial/office to residential without planning permission, within the CAZ |
| Class E / office to residential (outside CAZ) | 2021, confirmed 2022 | Same change of use rights, removed across the rest of the borough |
| Individual conservation areas (e.g. Queen's Park Estate, Sussex Gardens) | Varies | Minor works: painting, window replacement, boundary treatments |
The basement direction is the one most relevant to extension projects, since it applies to every property in the city regardless of conservation area status: no basement excavation in Westminster can rely on permitted development. For everything else - rear extensions, side infills, loft conversions - the starting question is the same: is the property in a conservation area, is it listed, and does any additional Article 4 direction apply. Only once those three questions are answered can you reliably work out whether national permitted development limits are even available to you. For a general primer on how permitted development rights have changed nationally, see our permitted development changes 2026 guide.
Basement Policy: Among the Strictest in London
Westminster was one of the first London boroughs to introduce a dedicated basement policy, responding to resident concern about the disruption caused by large-scale excavation projects beneath existing houses. The policy sits in the council's City Plan and is reinforced by the city-wide Article 4 direction covering all basement development since July 2016.
Key rules for new basements in Westminster
- Single storey only: City Plan policy limits new basement development to a single storey below the lowest original floor level, with departures from this only permitted in exceptional circumstances
- Maximum 50% garden excavation: no more than half the garden or site area behind the property can be built over by the basement
- No permitted development, ever: the borough-wide Article 4 direction means full planning permission is required for every basement scheme, with no exceptions for size
- Technical evidence required: applications need a structural methodology statement and a construction management plan addressing traffic, noise, and neighbour protection
This makes Westminster one of the two or three toughest London boroughs to secure a basement in, alongside neighbouring Kensington and Chelsea, which introduced a broadly similar single-storey policy after its own basement boom. Where above-ground extension options are already limited, a basement can still be the only realistic way to add significant space - but it needs to be planned around the single-storey and 50%-garden limits from the outset, not designed first and adjusted later. Our full basement extension guide covers costs, waterproofing, and party wall process in detail.
Planning Applications in Westminster: Fees and Timescales
When full planning permission is required - which, as above, covers the large majority of extension projects in Westminster - applications are submitted through the council's planning portal. The process itself follows the same national framework as any other London borough, but the assessment is more demanding given the weight placed on heritage and design quality.
Fees and determination times
Because so much of Westminster sits within a conservation area, applications are typically reviewed by both a planning case officer and a design/conservation officer, whose recommendation carries significant weight. A well-prepared heritage or design and access statement, addressing the specific character of the relevant conservation area rather than generic policy, materially improves the chances of a smooth decision.
Extension Costs in Westminster 2026
Westminster sits firmly in the prime central London cost bracket alongside Kensington and Chelsea, running 15-25% above the London average set out in our 2026 London extension costs guide. This premium reflects heritage material requirements (matching London stock brick, timber windows, slate or clay roofing), higher professional fees, and the additional cost of working carefully within conservation areas and around listed fabric. Applying that premium to the London averages gives a realistic Westminster range:
| Extension type | London average (all-in) | Westminster (all-in, +15-25%) |
|---|---|---|
| Single-storey rear extension | £60k–£135k | £70k–£170k |
| Two-storey rear extension | £110k–£290k | £125k–£360k |
| Loft conversion (dormer) | £52k–£100k | £60k–£125k |
| Basement (small, single storey) | £160k–£340k | £185k–£420k |
Note: figures are derived by applying Westminster's typical prime-central premium to the London-wide averages in our costs guide, not independent Westminster-specific project data. Conservation area and listed building projects should budget toward the top of each range.
| Extension type | London average (all-in) | Westminster (all-in, +15-25%) |
|---|---|---|
| Single-storey rear extension | £60k-£135k | £70k-£170k |
| Two-storey rear extension | £110k-£290k | £125k-£360k |
| Loft conversion (dormer) | £52k-£100k | £60k-£125k |
| Basement (small, single storey) | £160k-£340k | £185k-£420k |
Source: Derived from Mayfair Studio London extension cost data, 2026 • Data as of 2026
Additional costs specific to Westminster
Why Lawful Development Certificates Matter More in Westminster
A Lawful Development Certificate (LDC) is a formal, appealable determination from the council confirming that specific works are lawful without needing full planning permission - either because they fall within permitted development rights, or because existing works have stood long enough to become immune from enforcement. In most London boroughs, an LDC is a useful but optional safeguard. In Westminster, it is close to essential, because with more than three-quarters of the city inside a conservation area, around 11,000 listed buildings and structures, and a borough-wide Article 4 direction on basements, it is unusually easy to assume permitted development applies to a property when it does not. A boundary that clips the edge of a conservation area, a curtilage listing attached to a neighbouring building, or a street-specific Article 4 direction can all remove rights an owner might otherwise take for granted.
The consequences of getting this wrong are also more severe here than elsewhere: unauthorised works in a conservation area can trigger enforcement action, and unauthorised works to a listed building are a criminal offence, not just a planning breach. An LDC, priced at £274, converts an assumption into a legal determination the council cannot easily revisit later - whether confirming that a proposed extension qualifies as permitted development, or that an existing extension built years ago is now immune from enforcement under the four-year or ten-year rule. The sensible order of operations is: confirm conservation area, listed building, and Article 4 status first; establish whether any permitted development genuinely survives; and where it does, secure an LDC before committing to a build rather than relying on assumption.
Planning Tips for Westminster
Never assume permitted development applies
With 56 conservation areas covering more than three-quarters of the city, the safer default in Westminster is to assume full planning permission is required and be pleasantly surprised if it is not, rather than the other way round.
Check listed status even if the building looks unremarkable
Around 11,000 buildings and structures are listed across Westminster, including many ordinary terraced houses that form part of a wider listed group. Curtilage listing can also catch outbuildings and garden structures that are not separately listed.
Plan basements around the single-storey and 50% rules from day one
Do not design a basement first and try to fit it to policy afterwards. The single-storey limit and 50%-garden cap should shape the brief from the earliest concept stage, and a structural methodology statement should be commissioned early.
Budget for pre-application advice, and get an LDC where PD survives
Given the weight placed on design quality and heritage impact, pre-application advice is a sound investment on almost any Westminster extension. Where a property is confirmed outside a conservation area, unlisted, and free of any specific Article 4 direction, a £274 Lawful Development Certificate turns that assumption into a formal, defensible record before work starts.
Westminster has 56 conservation areas covering more than 76% of the city and around 11,000 listed buildings and structures, among the highest concentrations of any local authority in England. A borough-wide Article 4 direction has removed all permitted development rights for basements since 2016, and City Plan policy limits new basements to a single storey with no more than 50% of the garden excavated. Extension costs run 15-25% above the London average.
Frequently Asked Questions
Do I need planning permission for a house extension in Westminster?
In most cases, yes. More than three-quarters of Westminster falls within a conservation area, which removes or reduces permitted development rights for side extensions, roof alterations, and cladding, and limits rear extensions to smaller sizes than the national default. Even outside conservation areas, listed building status or an Article 4 direction can remove permitted development rights. Basements need planning permission everywhere in the city, with no exceptions, due to a borough-wide Article 4 direction in force since 2016.
How many conservation areas are there in Westminster?
Westminster has 56 conservation areas, covering more than 76% of the city according to the council's own published figures - one of the highest proportions of any local authority in England. Well-known examples include Mayfair, Belgravia, Marylebone, and St John's Wood.
How many listed buildings are there in Westminster?
Westminster City Council states that the statutory list includes around 11,000 buildings and other structures in the city, one of the highest totals of any single local authority in England. If your property is listed, you need listed building consent in addition to planning permission for most extensions, and often for internal alterations too.
What is Westminster's basement policy?
Westminster's City Plan limits new basements to a single storey below the lowest original floor level, with no more than 50% of the garden excavated. A borough-wide Article 4 direction, in force since 31 July 2016, removes all permitted development rights for basements, so every scheme needs full planning permission plus a structural methodology statement and construction management plan.
How much does a house extension cost in Westminster in 2026?
Westminster sits in the prime central London cost bracket, typically 15-25% above the London average. That gives roughly £70k-£170k for a single-storey rear extension, £125k-£360k for a two-storey rear extension, £60k-£125k for a loft conversion, and £185k-£420k for a small single-storey basement, all inc VAT and all-in.
What are the planning application fees in Westminster?
A householder planning application costs £548, the standard national fee in England. A Lawful Development Certificate costs £274. Westminster also operates a published pre-application advice fee schedule, priced by the scale of works, which is worth budgeting for given the borough's design expectations.
Why does a Lawful Development Certificate matter in Westminster specifically?
Because so much of Westminster is covered by conservation areas, listed buildings, and an Article 4 direction on basements, it is unusually easy to wrongly assume permitted development applies. Unauthorised works to a listed building are a criminal offence. A £274 Lawful Development Certificate gives a formal determination rather than relying on an informal assumption.
How long does planning permission take in Westminster?
Householder and other minor applications target 8 weeks, in line with the national standard. Applications involving listed buildings, heritage consultation, or planning committee referral typically take longer. Pre-application advice, priced on a published fee schedule, is strongly recommended before submitting in conservation areas or for listed buildings.
Summary
Westminster asks more of homeowners than almost any other London borough before they extend. With 56 conservation areas covering more than three-quarters of the city and around 11,000 listed buildings and structures, the assumption that permitted development applies simply does not hold in most cases. Basements face a further borough-wide restriction, limited to a single storey and 50% of the garden, with no permitted development route available at all.
None of this makes a Westminster extension unachievable, but it does make preparation essential: confirm conservation area, listed building, and Article 4 status before any design work begins, budget for pre-application advice and heritage-grade materials, and where permitted development genuinely does survive, secure a Lawful Development Certificate rather than relying on assumption. Costs sit at the premium end of London pricing, typically 15-25% above the London average, but property values in the borough mean well-executed extensions still deliver strong returns. The first step for any Westminster project is establishing exactly where your property stands against conservation, listed building, and Article 4 designations - that single question shapes every decision that follows.