Define what the extra room must achieve
Explain the activity the existing house cannot accommodate comfortably. That might be cooking and eating together, a better connection to outside space or a separate working area. Draw the furniture and movement needed for that use before selecting the extension's outline. Extra floor area can be disappointing if the original room becomes an awkward passage or loses a useful window. Compare the proposed addition with a careful rearrangement inside the existing envelope so the benefit of extending is clear.
The Northampton Square assessment identifies surviving Georgian houses. For an individual Clerkenwell house, that architectural context should inform how an addition meets the existing building, rather than dictate a generic extension shape.
Survey the plot and the rooms behind it
Measure existing levels, openings, walls and boundaries, including the relationship with adjoining properties where it can be established. Identify drains, external equipment and access points before placing a new footprint over them. Record which information is confirmed and which requires investigation. A garden photograph cannot establish the condition of buried services or the construction of a boundary wall. The feasibility drawings should identify those unknowns explicitly rather than showing a clean empty rectangle where the new work is intended.
Consider the retained outside area as usable space in its own right. Show furniture, circulation, planting and access for maintenance. An extension should not be judged only by the internal room it creates. Check how it changes the route to the outside and whether useful access remains beside or through the addition. This provides a practical basis for comparing schemes with different footprints.
Establish the permission route for this property
Islington's current application guidance distinguishes houses from flats. A house extension needs a householder application where it is not permitted development; an extension to a flat requires full planning permission. Listed-building consent is a separate consideration where applicable.
Confirm the property type and ownership before using that guidance. A ground-floor home with garden access may still be a flat. An extension drawing should therefore follow verified information about the premises rather than the appearance of its entrance. Check conservation designation, relevant directions and the proposed external changes against current sources. Keep the result with the scheme so that a later revision can be assessed against the same questions.
Treat an early concept as a design option until the necessary information and approvals are resolved. Record any constraints that require a different shape, material or junction rather than quietly removing them from the drawing. A smaller or differently arranged proposal may serve the brief better once those factors are understood.
Design the connection to the original house
The opening between old and new determines much of the extension's usefulness. Consider its width, position, remaining wall returns and relationship with furniture. Ask the structural designer to assess the proposal and establish the information needed about existing construction. A broad opening should not be specified on the assumption that the surrounding fabric can support it without further work. Coordinate structure with glazing, ceiling levels and the intended finishes before an attractive visual becomes a fixed expectation.
Light needs consideration on both sides of the connection. Test how the original room will be used after the addition and where daylight reaches its working surfaces. Roof glazing, doors and windows should form a deliberate arrangement rather than a collection of openings added to compensate for an unresolved plan. Their cleaning, maintenance and controls also need practical access within the completed space.
Resolve water, thresholds and services
Draw the threshold between inside and outside with the intended finished levels. Explain the relationship between door frame, paving and drainage, obtaining appropriate technical design for the junction. A low visual threshold does not remove the need to manage water. The outside surface and the interior floor should be coordinated before either is ordered. Include the space required for door movement and the treatment of corners, not just a typical central section.
Plan roof drainage and identify where it connects, then confirm the route and condition of the receiving arrangement. New kitchen or heating installations may also need connections through existing parts of the house. Make those routes part of the extension scope. State which existing rooms will be affected and how they will be made good, so that work beyond the new footprint is neither overlooked nor left undefined.
Prepare for construction and completion
Consider how people, materials and removed spoil reach the work area. If access is through the house, describe protection and the handling of large components. Identify the stages when existing rooms are opened to the addition and agree appropriate temporary measures with the responsible specialists. Retained rooms should not be treated as unlimited storage simply because they connect to the construction area. The working arrangement must be considered with the household's proposed use of the building.
Completion checks should include the connection to the original house and the retained outside space as well as the new room. Inspect door operation, threshold junctions, drainage access and controls. Gather the relevant installation information and record concealed routes. Review outstanding items by location and explain maintenance needs, including roof or glazing components that may require specialist access later. The final room should fulfil the original activity brief without making the rest of the house harder to use.
Before work begins
House extensions in Clerkenwell: questions
Are extensions suitable for every Clerkenwell house?
No area-wide suitability is claimed. The council's Northampton Square document confirms local house stock, but the individual plot, building, outside space and designation determine what can be considered. Start with a survey and the activity the extension must support. Compare options against retained rooms and garden use before assuming that the largest footprint is the best solution.
Does garden access make a Clerkenwell flat a house for planning?
No. Islington's guidance treats flat extensions separately and says they require full planning permission. Establish the property's actual type and the extent of ownership before choosing an application route. A private-looking entrance or garden does not settle those questions. The proposal also needs to address the building's shared construction and any relevant permissions from other parties.
Can an extension connect through a wide opening?
A structural designer needs to assess the existing construction and proposed opening. The design should coordinate support, remaining wall sections, ceiling levels and the intended door or glazing arrangement. Test furniture placement too, since a large opening does not automatically create a comfortable room. Confirm the feasible detail before ordering components that depend on it.
How is the remaining garden considered?
Show it as a usable area on the plan, with circulation, furniture, planting and maintenance access. Compare how different extension footprints affect those uses and the route from the house. The internal benefit should be considered alongside the outside space retained. A proposal that looks efficient as floor area can still be awkward when daily movement and upkeep are included.
What must be checked before selecting large doors?
Establish the structural opening, finished levels, threshold detail, drainage arrangement and route for delivery and fitting. Review opening movement and access for cleaning. The chosen system should fit the coordinated design, not become a fixed purchase that the building then has to accommodate. Keep product requirements with the drawings so installers can check the same dimensions and junctions.
Official sources
Northampton Square conservation design guidelines · Islington types of planning applications
Checked 4 November 2026. Maps, list entries and directions must be checked for the individual property and proposed work.