Rear extension (single storey)

Submission Readiness Score: 72/100 (Good)

Your application scores 72 out of 100, placing it in the “Good” category. This indicates that the fundamentals are strong, but there are specific areas where preparation can be improved before submission to maximise the chances of a smooth approval.

Score Breakdown

• Constraints (16/20): The site is largely unconstrained. The 4-point deduction reflects the TPO-protected oak tree, which requires an arboricultural assessment. Once this is commissioned, this score would rise to 20/20.
• Planning History (14/15): A clean planning history with no refusals. The minor deduction is for the 2023 withdrawn application, which, while not negative, leaves a small gap in the record.
• Project Fit (15/20): The proposed extension is well-suited to the property and its context. The deduction reflects the need to carefully manage the relationship with the attached neighbour at No. 16 and to confirm the extension depth will be proportionate.
• Documentation (13/20): This score reflects the current preparation stage. Once all drawings, the arboricultural report, and the application form are completed, this score will improve significantly. The documentation score rises as you complete the checklist items above.
• Policy Alignment (14/15): Strong alignment with local and national policy. The minor deduction reflects the need to demonstrate compliance with tree protection policies through the arboricultural assessment.

To raise your score above 80 (Excellent), focus on commissioning the arboricultural impact assessment and completing the full set of architectural drawings. These two actions alone would add approximately 10–12 points to your total score.

Refusal Risk Assessment

The overall risk of refusal for a single storey rear extension at 14 Rosewood Terrace is assessed as low. The property is free from significant planning constraints, the planning history is clean, and comparable applications in the immediate vicinity have been consistently approved.

The primary risk factors to manage are:

• TPO-protected tree (moderate risk): The oak tree in the rear garden is protected by a Tree Preservation Order. If the extension is designed without regard to the tree’s root protection area, or if an arboricultural assessment is not provided, the council’s tree officer may object. This risk is fully mitigable by commissioning an arboricultural impact assessment and designing the extension footprint to avoid the RPA.
• Neighbour amenity (low risk): As a semi-detached property, the relationship with the attached neighbour at No. 16 must be carefully managed. A single storey extension of 3–4 metres depth is unlikely to cause unacceptable harm to amenity, provided it does not project excessively beyond the neighbour’s rear wall and the boundary treatment is sensitively handled.
• Design quality (low risk): The council expects extensions to be sympathetic to the host dwelling in terms of materials, scale, and proportions. A flat-roofed single storey extension is an accepted and common form in this area, as demonstrated by the approval at 8 Rosewood Terrace.

There are no “red flag” issues that would suggest refusal is likely. The application should proceed with confidence, provided the above points are addressed in the design and supporting documentation.

Required Documents

May Be Required

Submission Checklist Summary

Your planning application for a single storey rear extension at 14 Rosewood Terrace requires 8 mandatory documents and 1 recommended supporting document. The council will not validate the application until all mandatory items are received.

The key item that is specific to this site is the Arboricultural Impact Assessment, which is triggered by the TPO-protected oak tree in the rear garden. This should be commissioned early in the process, as it may take 2–3 weeks for a qualified arboriculturalist to complete the survey and report.

All drawings should be prepared by a qualified architect or architectural technologist, drawn to recognised scales (1:50 or 1:100 for floor plans/elevations, 1:200 or 1:500 for the site plan), and clearly annotated with key dimensions, materials, and the property address.

Design Guidance for Single Storey Rear Extension

The following design principles should guide the development of the proposed extension at 14 Rosewood Terrace, informed by local policy requirements and the pattern of approved development in the area.

Scale and Proportions

The extension should be subordinate to the host dwelling in terms of scale, height, and massing. For a single storey rear extension to a semi-detached property, a depth of 3–4 metres from the original rear wall is well-proportioned and consistent with approved comparables on this street. The maximum height should not exceed 3–3.5 metres, with a flat roof or shallow mono-pitch being the most appropriate roof form for a single storey addition in this context.

Materials and Detailing

Merton’s design policies (Policy D4.2) require extensions to use materials that are sympathetic to the existing dwelling and the local streetscene. For 14 Rosewood Terrace, this means:

• External walls: Facing brick to match the existing dwelling, with matching mortar colour and pointing style. A sample panel may be conditioned.
• Roof: A flat roof with a parapet detail is the preferred approach for a contemporary single storey extension. Consider a sedum (green) roof to enhance biodiversity and manage surface water runoff — this is consistent with the approved scheme at 8 Rosewood Terrace.
• Windows and doors: Aluminium or powder-coated steel frames in a colour that complements the existing dwelling (e.g., anthracite grey). Bi-fold or sliding doors to the rear elevation are a common and well-accepted feature.
• Boundary treatment: Where the extension meets the shared boundary with No. 16, consider a set-back of 150–300mm from the boundary to allow for maintenance access and to soften the visual impact.

Neighbour Amenity

The relationship with the attached property at No. 16 Rosewood Terrace is the most sensitive design consideration. The extension should not project more than 1–1.5 metres beyond the neighbour’s rear wall at the shared boundary, unless the design incorporates a stepped-back or angled return that mitigates any sense of enclosure. Roof-level windows or rooflights should be positioned to avoid overlooking the neighbour’s garden.

Sustainability and Drainage

Consider incorporating sustainable design features such as high-performance insulation (exceeding Building Regulations minimum standards), energy-efficient glazing, and surface water attenuation. A sedum roof or permeable paving in the area affected by the construction will help manage surface water and can be highlighted in your application as a positive design response.

National Planning Policy Framework (NPPF) Alignment

The proposed single storey rear extension at 14 Rosewood Terrace is well-aligned with the relevant provisions of the National Planning Policy Framework (December 2024 revision). The key policy themes are design quality, residential amenity, and the protection of natural features.

Paragraph 135 establishes that plans should provide certainty about acceptable design. The London Borough of Merton’s Local Plan and supplementary design guidance set clear expectations for householder extensions, and the proposed development sits comfortably within these parameters.

Paragraph 139 requires development to be visually attractive and sympathetic to local character. A single storey rear extension in matching materials with a flat roof and contemporary detailing is a well-established and accepted building form in the Wimbledon area, as evidenced by the numerous approved comparables in the neighbourhood.

Paragraph 180 is relevant due to the TPO-protected oak tree in the rear garden. The NPPF places significant weight on the conservation and enhancement of the natural environment, including the protection of trees. The application should demonstrate, through an arboricultural impact assessment, that the development will not adversely affect this protected tree.

Overall, the NPPF supports the principle of well-designed householder development that improves the quality and functionality of existing homes, provided it respects local character and amenity considerations.