Does adding an extension or new floorspace change whether I cross the line?

No. The substantial-demolition test looks only at how much of the existing building you take down, measured by floor area. New floorspace you add does not count towards it. You can extend a Westminster home and still stay on the lighter side of the policy, as long as you keep more than half of the building that was already there. What you add is judged by other policies, not this threshold.

How do I show the council I am under the line?

Your application sets out the floor area retained and demolished, calculated to the standard. The council can secure the retained proportion with a planning condition, a binding requirement attached to your permission. So design in a buffer rather than to exactly half. Crossing the line later, during the build, would re-open the substantial-demolition tests and the audit that goes with them.

What does a Whole Life Carbon Assessment actually involve?

It measures the carbon of a building across its whole life. That means both the carbon built into its materials and the carbon of running it over time, calculated to a set standard. On a major Westminster scheme, it must also meet the council's own carbon limits. It is specialist work for your design team to commission, not a form you fill in yourself.

Is the policy in force now, or is there a transition for schemes already underway?

It has full weight from its adoption on 21 January 2026, and it applies to applications still to be decided. So a scheme not yet determined is judged against it now. No cut-off date for pipeline schemes has been published. Confirm how it is being applied with the council if you have a live application.

Most of my street is a conservation area. Does keeping the building guarantee approval?

No. Retention satisfies the retrofit-first test and counts in your favour, but it does not settle the heritage question. A building does not have to be individually listed to be controlled: sitting in a conservation area brings its own consent requirements. So a retention-led scheme can still be refused on conservation-area grounds if the design would harm the area. The two run in parallel.

Westminster Retrofit First: the 50% Demolition Rule

What changed in January 2026

On 21 January 2026 Westminster adopted Policy 43, called Retrofit first, as part of its City Plan, which is the statutory rulebook the council uses to decide planning applications. The policy is in force now, not a draft for consultation.

The principle is a reversal. All development should take a retrofit-first approach, which means reusing the existing building wherever possible, cutting embodied carbon, the carbon locked into a building's materials and construction, and reusing materials rather than sending them to landfill. For decades, demolition was the natural starting point. Now reuse is the assumption, and demolition is the thing you have to justify.

The council is careful to say that retrofit first does not mean retrofit only. Demolition is still allowed where it can be justified. The policy raises the bar, it does not close the door.

For a homeowner, the headline is short. A refurbishment that keeps the building's structure is largely unaffected by Policy 43. The weight of the policy falls on schemes that propose to demolish, because demolition now carries the burden of proof.

One point of scope before the detail. This is a Westminster policy. It covers Mayfair, Belgravia and Marylebone. It does not apply in Kensington or Chelsea, which sit under a different council with different rules. That contrast matters, and the last section returns to it.

The number that decides it, half the floor area

One measurement decides which version of the policy you face. Westminster calls the heavy trigger substantial demolition, and defines it as taking down 50% or more of a building's Gross Internal Area, on a building of more than one storey. Gross Internal Area, or GIA, is the total internal floor area of a building, measured to a set surveying standard so that everyone calculates it the same way.

Cross that line and you enter the demanding part of the policy. You must work through a staged test that makes you justify the demolition. You must also submit a Pre-Redevelopment Audit, a report that evidences why the building cannot simply be kept and reused. Both are explained in the sections below.

Stay below the line, by keeping more than half the floor area, and you avoid that branch of the policy entirely. The staged test and the audit do not apply to you.

The measurement is taken against the existing building, and in practice floor slabs are what you count. Because the figure can shift as a design develops, the council advises keeping a buffer of around 5% in the retained part, rather than designing to exactly half and risking a recalculation that tips you over.

What a structure-keeping refurbishment actually triggers

Here is the answer most homeowners want. If you gut and reconfigure a Westminster flat or townhouse but keep the structure, so that less than half the floor area is demolished, you do not trigger the staged test, and you do not need a Pre-Redevelopment Audit. You stay on the lighter side of the policy.

Two lower thresholds can still apply, and they are worth knowing before you settle a design.

The first is demolition above 10% of the floor area. Cross that and you need a Circular Economy Statement, a report setting out how materials will be reused and recycled, with an audit of what is taken out of the building. This is a Westminster threshold, and it is broader than the London-wide rule, which asks for such a statement only on the largest schemes.

The second is the size of the scheme. A major scheme, meaning 10 or more homes or 1,000 square metres or more of floorspace, needs a Whole Life Carbon Assessment, a measure of a building's carbon across its whole life, whatever the level of demolition. Most single-home refurbishments are not major. A large or multi-unit project can be.

For genuinely minor works that keep the structure, the usual document is a Sustainable Design Statement, a short account of the scheme's sustainability measures, rather than any of the demolition reports.

One honest caveat. Westminster has not published a single line confirming whether Policy 43 applies to ordinary home refurbishments. The position set out above is read off the policy's own thresholds, not stated directly by the council. Confirm your specific scheme through pre-application advice, the council's paid service for checking a proposal before you apply, before you assume where it lands.

What trips the line, and what justifying demolition takes

In a prime-London building, a few common moves cross the half-the-floor-area line. Keeping only a front façade and rebuilding everything behind it is one. Removing rear additions and enough internal floor structure that more than half the floor area goes is another. A deep retrofit, which the policy treats as replacing no more than half the existing floor slabs, stays on the retrofit side of the line.

If your scheme does cross the line, the policy asks you to work through a staged test, in a set order, set out in Westminster's Retrofit First guidance. You stop at the first stage you can satisfy. First, that the existing building is structurally unsound and cannot safely be retained. If you cannot show that, then that no retrofit, even a deep one, could meet a genuine design, access, statutory or operational requirement. If not that, then that demolition and rebuild would have lower whole-life carbon than a retrofit. And if not that, then that demolition delivers public benefits a retrofit could not.

The evidence for whichever stage you rely on goes into the Pre-Redevelopment Audit. The audit will not be accepted if it skips the order and jumps to a later test. On larger schemes, the council appoints an independent reviewer to check the audit, and the applicant pays for that review.

The plain consequence is that justifying demolition takes real time, real evidence and real fees. Retention is usually the quicker and cheaper route through planning. That is a reason to settle the question early, while the design is still flexible, not a flaw in the rule.

Heritage sits on top, not instead

Retrofit First is a separate question from heritage consent, and on most central Westminster schemes both apply at once. Around three-quarters of the borough sits within a conservation area, and there are over 11,000 listed buildings and structures, so a typical scheme in Mayfair, Belgravia or Marylebone engages heritage rules as well as Policy 43.

Keeping the building usually satisfies Retrofit First. The policy also gives real weight to retention when you are weighing the extensions or alterations needed to make a retrofit work, which can help an application. But listed building consent, the separate permission needed to alter a building of special architectural or historic interest, and conservation-area controls are statutory permissions in their own right. Policy 43 does not change them.

So a scheme can satisfy Retrofit First by keeping the building and still need listed building or conservation-area consent. It can also still be refused that consent on heritage grounds, if the design would harm the building or the area. The two assessments run in parallel. You clear both or you clear neither, and a strong retrofit case does not buy you a weaker heritage case. For how the wider system fits together, our guide to permitted development versus full planning sets out the routes.

What this means for your project

The practical upshot is straightforward. In Mayfair, Belgravia and Marylebone, design the scheme around keeping the structure unless there is a real reason to demolish. Cross the half-the-floor-area line only with your eyes open, because the proof it demands is slow and costly. If you cross the 10% demolition line, budget for a Circular Economy Statement. If the scheme is major or multi-unit, budget for a Whole Life Carbon Assessment whatever the demolition level.

Then there is the boundary. None of this applies in Kensington or Chelsea. They sit under a different council, which adopted a newer local plan with a softer approach: it prioritises retention but without Westminster's staged 50% test, and asks for whole-life carbon and circular economy work mainly on major schemes. The same kind of older townhouse can face two different rulebooks depending on which side of the borough line it sits. Check which council you are in before you plan to demolish.

There is a way this goes wrong that is worth naming. A team designs a demolition-led scheme first, then meets the retrofit-first requirements only at the validation stage, by which point the audit, the independent reviewer and the months of delay are already locked in. A construction manager who lets a client commit to taking down more than half a Westminster building, without first testing whether a retention-led scheme would clear planning faster and cheaper, is part of that problem, Myrmex included.

The policy rewards deciding early. Settle the demolition question against the half-the-floor-area line at the design stage, with your architect and a planning consultant, before the drawings harden. This is how the policy works, not advice on your own scheme. If you want to see how we work a project from the first decision, that is the next thing to read.

Westminster's Retrofit First policy (Policy 43): what it means for your refurbishment