Gatliff Road London Air space

The Challenge

Planning in Westminster: Adding an extra storey in Westminster required a well-justified proposal. The Gatliff Road location meant the additional floor had to respect surrounding building heights, local views and daylight levels. Any increase in height needed to be carefully explained through clear architectural drawings, accurate sections and a planning statement that demonstrated there would be no harm to neighbouring properties or the wider streetscape.

Existing RC Concrete Frame: The building’s reinforced concrete structure was not originally designed to carry another level above. Slab thicknesses, column sizes and existing load paths all limited what could be added. We needed to create a comfortable 2.5m internal head height on the new floor while working within these constraints and keeping floor and roof build-ups as lean as possible.

Mechanical & Fire Constraints: The client required a full mechanical redesign for the building, not just the new storey. New ventilation routes, risers and plant had to be threaded through tight ceiling voids and existing cores. These services could not break fire compartments, reduce the performance of fire-rated elements or compromise protected escape routes serving the whole building.

Our Approach

Detailed Architectural Drawings: AC Design Solution produced the full set of detailed drawings to support both the planning process and construction. Plans, elevations, sections and junction details were drawn to a high level of accuracy, with particular attention to overall building height, parapet lines and how the additional floor reads in longer views. All information was prepared in line with our building regulation drawings standards so that the planning package could flow directly into technical design.

Technical Coordination: We developed coordinated details showing how the lightweight structure for the new floor ties into the existing RC frame. Structural zones, insulation thicknesses and finishes were carefully balanced to keep build-ups efficient. Door and window heads, junctions with the parapet and interfaces with existing vertical circulation were all designed so that the internal spaces remained generous while still meeting regulatory requirements.

Mechanical Layout Design: Our team set out the mechanical layout for the building, including risers, plant locations and main distribution routes. We planned service runs in parallel with the architectural layout so that ductwork and pipework could be installed without awkward bulkheads or unexpected drops in ceiling height. This early coordination helped reduce clashes on site and gave the contractor a clearer route to installation.

Fire-Compliant Detailing: The drawings reflected the updated fire strategy, including fire-rated partitions, new compartment lines, protected routes and compliant service penetrations. Particular care was taken where new penetrations had to pass through existing slabs or walls. Our details helped the wider design team deliver a scheme that was not only safe and compliant on paper, but also buildable and inspectable on site.

How We Achieved the 2.5m Head Height

Optimising Floor & Roof Build-Ups: Achieving a 2.5m head height on top of an existing 18m building meant every millimetre counted. We reviewed alternative floor and roof build-ups, selecting combinations of structure, insulation and finishes that gave the required performance without unnecessary depth. This included considering lightweight construction options and carefully setting finished floor levels in relation to existing thresholds and stair landings.

Coordinating Structure and Services: We treated the ceiling zone as a shared space between the structure and the mechanical design. Early coordination between beams, ducts and pipework allowed us to avoid deep downstand beams or oversized services running across key rooms. This approach helped maintain clean ceiling lines and a consistent perceived height, which is especially important in commercial and mixed-use spaces where comfort and flexibility are key.

Future Flexibility in the Layout: As part of the planning and detailed design, we considered how the new floor might be used and adapted in the future. By keeping the structural grid regular and service routes logical, the new level can support different fit-outs over time without major alterations. This future-proofing helps protect the client’s investment and makes the building more attractive to future occupiers.

Results

Planning Permission Secured: Westminster City Council approved the application, satisfied that the additional storey respected the character and scale of the area. The planning officer’s report recognised that the new floor had been carefully integrated with the existing building and that the proposal was supported by clear and robust technical documentation.

New Floor with 2.5m Head Height: The completed design provides a valuable extra level on top of the 18m building, achieving the required internal head height while working safely with the existing reinforced concrete frame. The new storey reads as a natural extension of the original structure rather than an afterthought, both inside and out.

Coordinated Mechanical & Fire Strategy: The building now benefits from a modern mechanical system and an updated fire safety design aligned with current standards. Services, structure and fire protection have been coordinated so that the new floor and the existing building work together as a single, safe and efficient whole.

This Gatliff Road project shows how careful architectural design, detailed drawings and coordinated planning can unlock additional floors in central London while remaining compliant, efficient and buildable. It is a good example of how AC Design Solution support clients on technically challenging projects where planning risk, structural limits and building services all need to be balanced.

Thinking about extending upwards? Contact our architectural team at AC Design Solution to discuss detailed drawings, planning and technical design for your building.