Explain the need before drawing the outline
Identify the activity that cannot be accommodated comfortably in the current house. It may involve dining, family use, work or a better relationship with outdoor space. Compare that need with the potential of the existing layout before assuming an addition is the only answer. The first design exercise should show how the proposal improves the affected floor, including rooms that remain within the original building.
Test the options with furniture, entrances and circulation. An extension can change a previously well-lit room into an intermediate space, or move the main route through an area intended for quiet use. Those consequences need to be visible on the drawings. We discuss the trade-offs with the homeowner so the chosen scheme has a clear purpose, rather than treating the greatest possible footprint as the best result regardless of how the rest of the home changes around it.
Establish the site's actual limits
Obtain reliable information about boundaries, levels, existing construction and services. Clarify the relevant ownership and access arrangements with appropriate advisers. A garden or yard visible from the house is not, by itself, evidence that every part can be built over. The proposal needs to be based on the land and rights actually available, with any uncertainty resolved before a design depends on them.
Survey the relationship to neighbouring buildings and openings. Identify constraints that may affect construction access as well as the finished form. If a significant item remains concealed, record the investigation needed and the decision it could change. This gives the early design a realistic basis. It also avoids importing assumptions from a nearby project whose plot, level or approval history may be different despite a similar appearance from the street.
Read the proposal against Southwark's current guidance
Southwark's Householder Development SPD adopted in July 2025 addresses the design of additions and their effects on neighbouring amenity. The council's conservation register distinguishes local designations including Borough High Street, Bear Gardens and Union Street. The relevant property and boundary need to be checked before deciding how the proposal should respond to its setting. A shared SE1 postcode does not create a common extension entitlement.
Coordinate the planning advice with the developing design. The drawings should explain the scale of the addition, the position of openings and its relationship with surrounding spaces. Identify any assessment needed to support the proposal and allow its findings to influence the scheme. Where an option remains uncertain, make that clear to the homeowner instead of presenting a preferred sketch as if its acceptability has already been established by the existence of similar work elsewhere.
Sources: Southwark: adopted Householder Development SPD · Southwark: Householder Development SPD 2025 · Southwark: conservation-area register
Detail the new structure as part of the old house
The connection between existing and new construction needs coordinated technical design. Establish the structural approach with the relevant engineer and review floor, wall and roof junctions together. Drainage and service routes should be developed at the same stage. A narrow change in level or a retained opening can have a substantial effect on how the addition works, so these details should not be deferred until the new space is already being built.
Consider thermal, weathering and maintenance requirements alongside the visible materials. The design should explain how the junction performs and how equipment remains accessible after completion. Coordinate temporary conditions and protection of retained rooms with the construction sequence. If investigation reveals a different condition from the one anticipated, assess and record the response before continuing. A reliable extension depends on the existing building as much as on the specification of its newly constructed parts.
Review the changed home and retain its records
Inspect the addition from the existing house, the new room and the outside where accessible. Check transitions, door movement, controls and the practical use of the arrangement. Compare the finished materials and details with the agreed specification and record remaining items by location. The review should address the whole scope, including reinstatement around the connection, rather than regard the new room as complete while its meeting with the old house remains unresolved.
Keep the relevant approvals, final drawings and completion information together with product-care instructions. Identify access points for maintenance and explain the elements that remain existing construction. The owner should have a clear account of what changed and which documents describe it. That record supports routine upkeep and gives a future adviser useful information if later work affects the same structure, services or external junctions.
Before work begins
House extensions in Borough & Bankside: questions
Can a small addition be more useful than a larger extension?
It can be, depending on the problem being solved. A focused change may improve circulation or accommodate a specific activity without disrupting the rest of the house. We would compare options through their effect on everyday use and retained rooms, rather than assume that a larger footprint automatically creates a better home.
Does an existing garden guarantee room to extend?
No. Ownership, boundaries, services, neighbouring relationships and applicable controls need assessment. The design must respond to the actual site and the rights available to the owner. We would establish those matters before presenting the visible outdoor area as buildable space or developing a proposal that depends on an unresolved boundary assumption.
How is neighbouring privacy considered?
Southwark's guidance includes neighbouring amenity among the matters relevant to householder design. The position and form of proposed openings should be assessed against the actual surrounding buildings and spaces. We would coordinate the required planning advice and supporting information, allowing those findings to shape the proposal before its external arrangement is treated as fixed.
When are materials and roof details chosen?
They should develop alongside the spatial and technical design. Their appearance, construction requirements and junctions can affect the proposal and its relationship to the existing house. We would coordinate those choices with the relevant advice rather than leave them as decorative selections after the main form has already been submitted or instructed for construction.
What if opening-up changes the structural assumptions?
The affected work needs assessment by the appropriate professional before proceeding on the original basis. Record the condition and coordinate the revised detail with the other trades. We would update the relevant instructions so the extension is built from a considered response to the actual structure, rather than an informal adjustment to an earlier assumption.
Official sources
Southwark: adopted Householder Development SPD · Southwark: Householder Development SPD 2025 · Southwark: conservation-area register
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.