Establish whether there is a viable place to extend
The first investigation concerns the available building and land, not a preferred extension shape. We review the measured site, ownership information and existing access, identifying any shared areas or equipment that the proposal could affect. A courtyard visible from a window is not automatically part of the dwelling's usable development space. Where the boundary or rights are uncertain, the appropriate professional advice is identified before the design treats the area as available.
We then compare the proposed addition with the problem it is intended to solve. A dining space may work better through an internal rearrangement, while an extension can be justified where it creates a useful connection that the existing plan cannot provide. The comparison includes circulation, storage and the effect on rooms left behind. Additional floor area should not leave the original centre of the house dark, awkward or dependent on passing through another room for every daily activity.
Confirm the building type and planning context
The City Corporation distinguishes flats from houses when explaining residential planning rights. A dwelling's position at ground level does not by itself establish a householder route. We confirm the property type and proposed work before relying on national permitted development provisions. Those provisions contain conditions and limitations, and separate requirements can still apply. The proposal is described accurately so that the authority can assess the relevant alteration rather than an assumed category.
Conservation mapping and the relevant character guidance are checked for the location. The design review considers how the addition would relate to the existing building and its surroundings, including the parts visible beyond the property itself. We do not infer permission from a nearby extension. Its history, circumstances and approved design may differ. The useful question is what evidence supports this proposal, on this site, with its actual effect on the host building and neighbouring spaces.
Sources: City Corporation: residential planning permission · City Corporation: conservation maps and character documents · Government householder permitted-development technical guidance
Design the opening and the new room together
The connection between old and new often determines how an extension feels. We consider the width and position of openings alongside the remaining wall, furniture arrangement and circulation route. The structural designer assesses the proposed load path and support requirements. Any necessary investigation is identified before a large opening becomes a fixed assumption. A drawing that shows a clear space without explaining how it is supported does not provide a construction solution.
Floor levels, roof junctions and drainage routes are coordinated in section as well as plan. The detail should explain how water is kept out, how the existing structure is protected and how the new enclosure can be maintained. We review the effect of the proposed roof and glazing on the rooms behind it. Large areas of glass may change glare, heat and privacy, so they are assessed as part of the room's use rather than as a default way to make an addition feel larger.
Check neighbours and construction access early
The assessment includes relevant adjoining windows, boundaries and outdoor spaces, with specialist studies identified where the proposal requires them. We avoid treating a plan-view distance as the only measure of neighbour impact. Height, orientation and the shape of the surrounding buildings can influence the questions that need investigation. The design record explains the options considered and why a particular arrangement is being pursued, making the reasoning available for the application and later technical design.
Access is tested against the construction method. We identify how materials, equipment and waste can reach and leave the work area, and whether temporary arrangements require agreement with others. The sequence accounts for protection of retained rooms and essential services. A constrained site may influence component sizes and the order in which work is assembled. These practical choices belong in the developed proposal, because a scheme that cannot be delivered responsibly is not ready merely because its elevations look resolved.
Define the complete extension, including the old rooms
The specification covers the addition and the work it triggers in the existing home. That includes making good, changes to services, altered finishes and the treatment of the threshold between old and new. We clarify whether adjoining rooms are being redecorated or retained, and where the boundary of each trade's work lies. The aim is to prevent the new space being fully specified while its connection to the house remains a collection of assumptions.
During completion we review the enclosure, openings, controls and access for maintenance, alongside the agreed internal finish. The owner receives relevant installation and completion records, with the final arrangement reflected in the handover information. We also test the space against the original brief: how it is entered, furnished and used with the rest of the home. An extension should be understood as part of the building, not as an isolated room that happens to have been attached to it.
Before work begins
House extensions in City of London: questions
Can a City ground-floor flat have an extension?
It requires an individual assessment of ownership, structure, shared interests and the approval route. The Corporation cautions that rights applying to houses cannot simply be assumed for flats. A private-looking outside area is not sufficient evidence that it can be built over by the flat owner.
Does a neighbouring extension establish that mine will be approved?
No. The neighbouring property's permissions, dimensions and context may differ. We use relevant information as part of the research, but the proposed extension needs its own assessment. The design should address the conditions and effects of your site rather than copying an outcome elsewhere.
When is the structural engineer involved?
Structural input is needed while the design is being developed, particularly where the proposal changes openings, loads or foundations. The extent of investigation follows the existing building and proposed work. The room layout and structural solution should develop together rather than being reconciled after design decisions harden.
Can an internal reconfiguration be compared with extending?
Yes. We can compare how each option meets the brief, including daylight, circulation, storage and the effect on existing rooms. That comparison helps establish whether additional space is necessary and useful. It also makes clear which improvements depend on the extension and which do not.
What happens if access is too restricted for the initial design?
The construction approach and design are reviewed before the scheme is committed. Smaller components, a different sequence or a revised form may be considered, subject to the relevant technical advice. If a workable method cannot be established, that limitation is recorded rather than concealed in the proposal.
Official sources
City Corporation: residential planning permission · City Corporation: conservation maps and character documents · Government householder permitted-development technical guidance
Sources checked on the dates recorded in the source register, most recently 5 November 2026. Maps, list entries and directions must be checked for the individual property and proposed work.