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The flat roof of Collingwood House, Islington, as found: white roof membrane, old steel plant frames and the north London skyline under grey cloud
Case study: rooftop extension, pre-application

Collingwood House, Islington

Rooftop extension on an occupied building: one- and two-storey options tested at pre-application

Appraisal target, not a result

£3.9M target GDV for five
new apartments

Live project: pre-application stage

  • 2options drawn: one storey and two
  • 5apartments in the one-storey scheme
  • 9apartments in the two-storey option
  • 18 mheight line that changes the fire rules

Can this roof carry new homes? Find out before the full application

The freeholder of Collingwood House, a property investment company, owns a four-storey block on Mercers Road in Islington: a gym below, 13 flats above and a wide flat roof. The question was simple. Could a rooftop extension of one or two storeys add new homes?

The answer was not simple. The council had advised against a two-storey rooftop scheme here in 2016 and again in 2017. The building is in a conservation area and dates from 1938 to 1939, so there is no permitted development shortcut: it needs full planning permission.

So we did not start with a full application. Craftex assembled and manages the whole consultant team as the owner’s single point of contact, and split the work in two: first prove there is a scheme, then draw it and ask the council. This is a live project. Nothing on this page is a planning result.

The red-brick upper storeys of Collingwood House seen from the side yard, with the flat parapet line the rooftop extension would sit behind

The building as found

Built
1938 to 1939
Height today
Four storeys
Planning context
Conservation area
In use throughout
Gym, 13 flats

Own a building with an unused roof?

Send us the address. A director looks at the planning route, the height and the likely structure, and tells you whether a rooftop scheme is worth testing. Free, and we come back to you the same working day.

Rooftop extension drawings: one storey and two, tested side by side

Concept drawings for both options, prepared for the pre-application. They show volume and layout, not final architecture. Our goal for the owner is at least five two-bedroom apartments; the appraisal puts the value of five new apartments at about £3.9M. That figure is a target, not a result.

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Section: one new storey

One new floor on the existing roof slab, set back from the parapets. The existing stair is carried up to serve it.

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Section: two new storeys

A second, smaller floor above the first. This is the option that takes the building across the 18 m line.

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Fourth-floor plan: five apartments

The one-storey scheme: five apartments around the two existing cores, with terraces and a green roof on the set-backs.

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Front elevation: two storeys

The two-storey option from Mercers Road: about nine apartments and roughly 575 sq m of new floor space.

Upward extension on Mercers Road: existing and proposed

Our planning review found seven upward extensions approved within about 300 m. The pattern is consistent: set-back, recessive additions are approved; dominant ones are refused. So the design sits behind the existing parapet and reads as a new crown to the building, and the heritage case is written around that.

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ExistingFront elevation today

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Proposed, one storeyOne set-back storey (concept)

Want to see what your roof could hold?

We draw the options, count the homes and show you what each one triggers in planning, fire safety and structure, before you commit to an application.

The building and its history: a 1938 warehouse, altered for eighty years

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Built as a tobacco warehouse

Collingwood House was built in 1938 to 1939 as a tobacco company’s headquarters: a warehouse and loading bay on the ground floor, offices and a boardroom above. Even then the floors stepped back at different levels.

The section shows it today. No two parts of the roof are quite the same height, which is why nobody could trust a single desktop figure for this building.

View down from the roof parapet of Collingwood House: brick rear wings, a later grey-clad top storey and a lower gravel roof between them

Extended again and again

The building has been added to in almost every decade: at the rear in the 1950s and 1960s, at third-floor level around 1989, and at first floor around 2020. A warehouse became offices, then a health club, then flats.

From the roof you can read it: older brick wings, a later clad storey, and lower roofs between them.

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Old frame, new steel

Past surveyors did not even agree on what holds it up. One record calls the frame steel; a later engineer called it concrete. Our structural engineer walked the building, read the 1938 drawings in the archive and marked up each floor: original structure in blue, later additions in red.

What this means for you: on an old, much-altered building, the history is the first survey.

Extensions, changes of use and earlier rooftop attempts

From the Historic Building Report (December 2018), the structural engineer’s report (November 2026) and the planning record.

WhenWhat changed
1938 to 1939Built as a tobacco company’s headquarters: warehouse on the ground floor, offices above
1946Single-storey extension permitted to the south (since demolished)
1950 to 1966Rear extensions at first and second floor; third floor turned into a canteen
1953Roof assessed for staff recreation use; the slab and framing were judged adequate for it
1982 to 1986Warehouse closed; two applications to convert the whole building to flats refused at appeal
1988 to 1990Refurbished for business use: third-floor extension, rear extension, parts demolished
1998Ground, first and part of the second floor converted to a health club
2014 to 2015Parts of the second and third floors converted to flats
2016 and 2017Council gave adverse pre-application advice on a two-storey rooftop scheme
Around 2020Single-storey extension at first floor for the gym and its plant
2026Rooftop extension taken to pre-application by Craftex (this project)

Craftex is also delivering a separate project in the same building: the former gym space on the second floor is being converted into four flats.

Airspace development risks: what decides a scheme like this

Six questions decide whether this roof gets built on. Planning is only one of them.

QuestionWhy it matters hereWhat we do about it
Planning routeConservation area, built before 1 July 1948: no permitted development route. Two adverse pre-application responses on record.Policy and precedent review first; both options tested at pre-application before a full application.
Height and fire safetyOn desktop data the building is about 12.4 m: about 15.5 m with one storey, about 18.6 m with two. Over 18 m it becomes a higher-risk building.Measured survey of the real heights before the storeys are fixed. We are still working through the fire constraints of a building above 18 m.
ScaleAt around ten homes a scheme becomes major development: planning committee, a full Section 106 agreement and 45% on-site affordable housing.Unit numbers, Section 106 and the Community Infrastructure Levy (CIL) assessed for each option, so the owner sees the cost of each threshold.
StructureA 1938 frame with decades of additions. Nobody had established what the frame and foundations could take.Structural feasibility for one storey and two, assessed separately.
A building in useWorks above an operating gym and 13 occupied flats, on a tight site with restricted crane access.Buildability reviewed with the engineer from the first stage: lightweight construction, access and sequence.
Legal titleMany registered interests in one building. Airspace and Landlord and Tenant Act 1987 rights, lease terms, restrictive covenants, rights of light, party walls, crane and access.A risk list handed to the owner’s solicitors at the start, so legal work runs alongside planning.

The pattern: every one of these can stop the scheme, and each can be answered at modest cost before the expensive work starts.

Which of these could stop your scheme?

Tell us about the building. We go through the same six questions with you and say plainly where the risk sits.

Pre-application advice done in the right order: de-risk first

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Spend a little to find out, before a lot

Stage 1 answers three questions. A measured survey fixes the real heights. A structural feasibility study asks whether the frame and foundations can carry the load. A planning review tests policy and precedent. Each stage is paid in advance, and if Stage 1 shows the scheme does not stand up, the commission stops and no further fees are due.

The engineer’s feasibility report came back in November 2026. On its assumptions, one lightweight storey can be carried by the original frame without strengthening, subject to site investigation. A second storey is a bigger structural step: the report sets out the extra support and testing it would need. Areas already extended upwards are treated with caution, as the overlay shows.

What this means for you: you know whether you have a scheme before you pay for one. Our development feasibility study works the same way.

Where the project stands: pre-application in progress

An honest status, as of November 2026. We will update it as the project moves.

StepStatus
Stage 1: survey, structural feasibility, planning reviewDone; structural feasibility report issued November 2026
Stage 2: concept design for one and two storeysDrawn (the drawings on this page)
Pre-application documentsReady to submit, with a request for a meeting with the council
Meeting with the councilNot yet held; we expect it two to three weeks after submission
Written advice from the councilNot yet received; we expect it four to six weeks after the meeting
Five-apartment, one-storey schemeBeing prepared in parallel and assessed financially: CIL, Section 106, value and build cost
Viability assessment and full planning applicationNext stage; not started

There is no statutory timeframe for pre-application responses, so the dates above are expectations, not promises.

Start with the stage that can say no

A staged proposal with a clear scope for each consultant, a list of what is not included, and a stop point after the first stage. One team and one point of contact from the first survey to the planning decision: that is our development management service.

Our team

  • in

What Craftex delivers at pre-application

One appointment instead of five. The owner deals with Craftex; we brief, coordinate and answer for the team. This is how our planning consultants run a difficult site.

DeliverableWhat the owner gets
A multi-disciplinary teamPlanning consultant, structural engineer, architect and measured surveyor, coordinated by our planning director
One staged proposalA clear scope for each consultant, an exclusions list and a staged payment schedule with a stop point
Planning strategyPolicy and precedent review, and the heritage case for a set-back design that re-crowns the building
Risk list for the owner’s solicitorsAirspace and leaseholder rights, lease terms, covenants, rights of light, party wall, crane and access
Measured surveyHeights and levels of the building and its neighbours
Structural feasibilityOne-storey and two-storey options assessed separately
Pre-application packageConcept massing for both options, schedule of accommodation, white-box 3D images, initial Design and Access and Heritage Statement, and the submission
A single point of contactOne director who reports, chases and recommends whether to go on

Free value: take this with you

Twelve checks before you commission a rooftop extension

  1. Before you spend on design

    • Check the permitted development route first. A conservation area or a building finished before 1 July 1948 closes it, and full planning is a different budget.
    • Pull the planning history. Earlier refusals and pre-application letters on your building tell you what the council has already said no to.
    • Read the precedents on your street. Which rooftop schemes nearby were approved, which were refused, and what the decision notices say about set-backs.
    • Find the building’s own history. Old extensions and changes of use decide what the structure can take.
  2. Before you fix the number of storeys

    • Measure the real height. Desktop figures are not enough when 18 m changes the fire-safety regime for the whole building.
    • Get a structural feasibility view for each option. One storey and two are rarely the same question.
    • Count the homes against the thresholds. Ten homes is major development: committee, Section 106 and on-site affordable housing.
    • Plan the build over people. Occupied flats, a trading ground floor and crane access all shape cost and programme.
  3. Before you talk to the council or the leaseholders

    • Send the title to your solicitors early. Airspace rights, lease terms, covenants and rights of light can stop a scheme that planning would allow.
    • Take legal advice before any disposal of the airspace. Leaseholders may have statutory rights, and the notices must be served correctly.
    • Go to pre-application with a scheme that has survived its own tests. Survey, structure and policy first; drawings second.
    • Stage the fees. Agree in writing that the work stops, with nothing more to pay, if the first stage says no.

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Rooftop extension and pre-application: your questions

Do you need planning permission for a rooftop extension?

Usually, yes. Some buildings can add storeys under permitted development with prior approval, but that route is closed to buildings in a conservation area and to buildings completed before 1 July 1948. Collingwood House is both, so it needs full planning permission. We check the route in a free site appraisal before any design money is spent.

What is pre-application advice, and is it worth it?

It is the council’s written view on a scheme before a full application is made. On a site with a difficult history it is worth it: you learn what officers will support for the price of concept drawings, not a full application. At Collingwood House the council had already advised against a two-storey rooftop scheme twice, so we test both options first.

Can you build a rooftop extension in a conservation area?

Yes, with full planning permission and a design that respects the street. Near Collingwood House seven upward extensions have been approved within about 300 m. The ones approved were set back and quiet; the ones refused dominated the building below. A heritage statement explains why the design fits.

Can an existing building carry an extra storey?

It depends on the frame and the foundations, and an engineer has to check both. Here the structural feasibility report found that one lightweight storey can sit on the original frame without strengthening, provided site investigation confirms the assumptions. Parts of the building that were already extended upwards are treated more cautiously.

How many flats can a rooftop extension add?

It depends on the roof area the structure can carry and the height the street will accept. At Collingwood House the one-storey scheme is five apartments and the two-storey option is nine. From ten homes a scheme becomes major development, with on-site affordable housing and a Section 106 agreement.

What is airspace development?

Building new homes on top of an existing building, using the unused space above its roof. The owner keeps the building and gains new flats to sell or let. It depends on four things being true at once: planning, the structure, fire safety and the legal rights over the roof.

How long does pre-application advice take?

There is no statutory timeframe. On this project we expect a meeting about two to three weeks after the request is lodged, and written feedback four to six weeks after the meeting. Councils differ, so treat these as working assumptions.

What happens when a building goes over 18 metres?

A residential building of 18 m or more, or seven storeys or more, is a higher-risk building. It falls under the Building Safety Regulator, with stricter fire-safety design and approval gateways before and after construction. That adds time and cost, so the real height is measured before the number of storeys is fixed.

Do leaseholders have a say in an airspace development?

They can. Under the Landlord and Tenant Act 1987, qualifying leaseholders may have a right of first refusal when a freeholder disposes of the airspace. Lease terms, restrictive covenants, rights of light and party wall matters also apply. We list these risks for the owner’s solicitors at the start; the legal advice itself is theirs.

Has the Collingwood House rooftop extension got planning permission?

No. This is a live project at pre-application stage. The pre-application documents are ready to submit and the council has not yet given its view. We will update this page as the project moves; nothing here is a planning result.

Book a call with our team

Send the address and where you stand. A director reviews your building free, with no obligation, and replies the same working day. Planning advice, design, cost plan and programme come from one accountable team, from first appraisal to rental-ready handover.

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info@craftex.co.uk

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