Define the existing space and the proposed change
We begin by recording floor levels, access, visible construction and the current use of the lower-ground area. The survey distinguishes an existing room from a void or service space and identifies where its boundaries remain uncertain. Ownership and rights to alter the ground are reviewed through the appropriate advisers. A cellar reached from a dwelling may still have constraints that affect what can be done there, so access alone is not treated as evidence of an unrestricted development opportunity.
The brief then states the intended change precisely. Repairing and refitting within the current enclosure differs from lowering a floor, enlarging an opening or extending beneath another part of the site. Each can affect the investigations and approvals required. We compare the proposed use with available daylight, circulation and service arrangements before drawing a finished room. That helps establish whether the desired accommodation is realistic and which limitations must be resolved before the design develops further.
Investigate archaeological implications without assuming the outcome
The City's archaeology guidance calls for early discussion about the scope of assessment where development may affect archaeological interest. Its assessment process considers existing basement levels, foundations and previous disturbance alongside evidence of surviving remains. That information can influence the form of an excavation or its foundation design. We identify the appropriate specialist input at the feasibility stage rather than treating archaeology as a document to obtain after the basement shape has been fixed.
The guidance does not support a universal claim that basement work is impossible or that every property requires the same fieldwork. The relevant investigation follows the site and proposal. Our project brief distinguishes the archaeological question from the structural and water-management questions, while allowing their findings to inform each other. Where evidence is incomplete, the uncertainty is retained in the decision record. It should not disappear merely because a proposed room has been drawn convincingly.
Sources: City Corporation: archaeology and development guidance
Coordinate ground, structure and water management
The structural and ground investigations are defined by the relevant specialists for the actual building and proposed work. We record how their findings affect floor levels, supports and the extent of excavation under consideration. Temporary works and the construction sequence require their own design attention; the final room layout does not explain how the building is supported while it is being created. The proposal develops around those requirements rather than assuming a standard underpinning method.
Water management is considered as a complete system, including the existing conditions, proposed construction, drainage and future maintenance. The design should explain interfaces and access to equipment, with responsibilities made clear between specialists and installers. We avoid treating a product label as a guarantee that any space can be used for any purpose. An unresolved drainage or maintenance arrangement can affect the suitability of the proposed room, even if the decorative design appears complete.
Assess the intended room and its route to the home
The Corporation distinguishes residential alterations to flats from those to houses, and listed-building requirements may also affect the proposed work. We establish the property type and relevant designation before defining the consent route. A lightwell, altered opening or removal of historic fabric is identified explicitly. The authority needs to understand the actual change, while the owner needs to understand that an existing below-ground area does not automatically permit enlargement or a different use.
The layout is tested in section for usable headroom, the stair and its connection to the floor above. We review daylight, ventilation and the technical requirements associated with the intended use with the relevant designers. Storage or utility accommodation presents a different brief from a regularly occupied living room. The design should describe the proposed use honestly and account for it throughout, rather than calling a space storage in one document and presenting it as a bedroom in another.
Sources: City Corporation: residential planning permission · City Corporation: listed buildings and alterations
Prepare for construction and continued maintenance
We assess how excavated material, new components and equipment can be moved through the property without relying on an assumed access route. Protection of retained areas, temporary service arrangements and the effect on other occupants are considered in the construction plan. The method must reflect the actual site and specialist design. Where a practical route cannot be established, that finding can change the proposal before the owner commits to a scheme that is difficult to deliver.
Completion information records the installed systems, relevant inspections and the access needed for future servicing. We review the room's operation and its connection to the rest of the home, including doors, controls and maintainable equipment. The handover distinguishes completed work from outstanding matters. If the feasibility evidence does not support the proposal, we report that clearly. Basement assessment is available across the City service area, but neither excavation nor a particular room use is promised for every property.
Before work begins
Basement conversions in City of London: questions
Can an existing cellar simply be called a habitable room?
No. Its suitability depends on the intended use, condition, access and relevant technical and approval requirements. We assess those matters before presenting it as living accommodation. An existing enclosure is useful evidence, but it does not settle the acceptability of every proposed use.
Is every City basement excavation prohibited?
The official archaeology guidance describes a site-specific assessment process, not a blanket prohibition. Archaeological evidence may affect design and investigation. Other structural, planning, ownership and water-management issues also need examination before a conclusion can be reached about an individual proposal.
Do you promise a basement beneath a leasehold flat?
No. The ownership of the space and ground, the building's structure and shared interests must be established first. We identify the information and professional advice needed. A ground-floor position does not itself give the occupier the right or physical ability to excavate below it.
When should water-management design begin?
It should form part of developing the proposal, alongside structure, drainage and intended use. The design needs coordinated interfaces and a practical maintenance arrangement. Leaving those decisions until finishes are selected can result in a room layout that cannot accommodate the necessary construction or equipment.
What happens if the investigation does not support excavation?
We record the reasons and can assess a reduced scope, repair of existing lower-ground space or improvements elsewhere in the dwelling. The decision follows the evidence. We do not substitute a more confident description for unresolved conditions simply to keep the original room proposal intact.
Official sources
City Corporation: archaeology and development guidance · City Corporation: residential planning permission · City Corporation: listed buildings and alterations
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.