Designing Extensions in Conservation Areas: What You Need to Know

A Conservation Area designation brings several practical changes to what you can do without planning permission and how planning applications are assessed:

• **Reduced Permitted Development (PD) rights**:
• Within a Conservation Area, several types of work that would normally fall under PD rights require a planning application instead:
• •Cladding the exterior of a house in materials that change its appearance (e.g., rendering a brick exterior) — requires planning permission
• •Side extensions visible from a highway — require planning permission (rather than PD)
• •Any changes to the front elevation visible from the highway — require planning permission
• •Some rear extensions may also require planning permission depending on borough-specific Article 4 Directions

**How Conservation Area planning officers assess design**: When reviewing a planning application within a Conservation Area, the officer will assess: 1. **Does the proposal preserve or enhance the character and appearance of the Conservation Area?** (The test under Planning Policy Framework and Section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990) 2. Is the proposed extension **subordinate to the original dwelling**? (Extensions that dominate or overpower the host building are typically refused) 3. Are the materials **appropriate to the setting**? (Stock brick, clay roof tiles, timber sash windows are generally favoured over materials that contrast sharply with the surrounding character) 4. Does the design **respond to the established pattern of development** in the street or terrace?

**Conservation Area Appraisals**: Most London boroughs have published a Conservation Area Appraisal for each designated area — a document that describes the character, history, key views, and materials of the area. These appraisals are a valuable source of information about what the council values in that area. Read yours before commissioning design work.

**Principle 1: Subordinate at the rear, invisible or minimal at the front** The most consistently approved approach to extensions in Conservation Areas is to concentrate the additional space at the rear (away from the highway and from the primary views of the Conservation Area character), keep any side extensions set back from the main front elevation, and avoid any changes to the front of the property.

Rear extensions in Conservation Areas are routinely approved because they are not visible from the street — the Conservation Area character is preserved because the street view remains unchanged.

• **Principle 2: Match or complement materials**
• In most London Conservation Areas, the character buildings are stock brick Victorian or Edwardian terraces. Planners generally expect new extension materials to either:
• •**Match** the existing material (e.g., reclaimed or carefully matched new stock brick, matching clay roof tiles)
• •**Clearly contrast** as a deliberate contemporary design statement (e.g., a fully glazed, frameless extension that makes no attempt to imitate the original material but reads as a clearly modern addition)

The worst approach — and the one most often refused — is a half-hearted imitation: a brick that almost matches, render applied to look like stone, or concrete tiles where clay tiles were specified. A confident contemporary approach or a careful material match is safer than an awkward hybrid.

**Principle 3: Respect the roofline and ridge line** Extensions that break the established roofline of a terrace — by exceeding the ridge height of the host building, or by using a dormer design that projects significantly above the existing eaves — are commonly refused in Conservation Areas. A parapet or low-pitch flat-roof extension that sits below the eaves line of the existing building is far easier to approve.

**Principle 4: Retain original features on the host building** Planners look at the whole picture — not just the new extension. If the application also proposes removal of original features (sash windows replaced with casements, removal of chimney stacks, removal of original railings), these will attract additional scrutiny and may be conditions of any approval. Retaining original features on the host building significantly strengthens the overall application.

**Principle 5: Use glazing as a design device, not a default** In Conservation Areas where the street character is solid masonry construction, a large proportion of new frameless glazing in the extension can read as incompatible. However, a disciplined, high-quality glazing system — slim aluminium or steel frames, structural glass to a courtyard or garden — can be approved where it reads as a contemporary but considered response. Present it confidently and deliberately in the design statement, not as a cost-saving measure.

• **Pre-application advice (pre-app)**:
• For Conservation Area extensions, pre-application advice from the local planning authority is strongly recommended. It costs £150–£2,500 depending on the borough and the scale of the proposal, but gives direct feedback on:
• •Whether the principle of development is likely to be acceptable
• •Material preferences
• •Design approach concerns
• •Whether a heritage impact assessment is required

Pre-app advice is not binding, but a written pre-app response with a positive view significantly reduces refusal risk and can strengthen the application in the event of an appeal.

• **Heritage Statement / Design and Access Statement**:
• For Conservation Area applications, the local planning authority will typically require a Design and Access Statement (DAS) that:
• •Describes the history of the property and its relationship to the Conservation Area
• •Explains the design process and principles
• •Sets out how the proposed extension responds to the character of the Conservation Area
• •Lists the materials proposed and why they were chosen

A well-written DAS that directly addresses the Conservation Area Appraisal criteria significantly improves approval prospects.

**When is a Heritage Impact Assessment (HIA) required?** For larger or more complex proposals in Conservation Areas — especially those near listed buildings, in a setting where the impact on a listed building must be assessed, or where the Conservation Area Appraisal identifies key views that the development may affect — the LPA may require a Heritage Impact Assessment from a conservation consultant. This is in addition to, not instead of, the DAS.

**Common reasons for refusal in Conservation Areas**:

1. **Loss of front garden / changes to front boundary**: Removing boundary walls, railings, or front gardens for parking in Conservation Areas is frequently refused — front boundary features are often specifically identified as character elements in the Conservation Area Appraisal.

2. **Dormer or roof extensions visible from the highway**: Dormers that project above the ridge, or that are visible from the primary street, are refused far more often than dormers set back from the main roof plane and visible only from the rear.

3. **Materials clearly at odds with local character**: uPVC windows in a Georgian terrace, white render on a stock-brick Victorian street, flat concrete tiles on a Conservation Area with a clear character of clay pantile roofs.

4. **Extensions that exceed the scale of the original building**: A rear extension that clearly dominates the original house in plan or volume is likely to be considered harmful to the character of the original building even if it is not visible from the street.

5. **Lack of a considered design rationale**: Applications where the design statement does not engage with the Conservation Area Appraisal or simply asserts that the proposal 'preserves the character' without evidence are treated with lower credibility by officers.

**What to do if refused**: If an application is refused, the reasons for refusal may be curable — in which case a pre-application meeting to discuss what a revised scheme would need to address, followed by a revised application (which can be submitted within 12 months without a second application fee), is the appropriate route. If the refusal is based on principle rather than detail, an appeal to the Planning Inspectorate is possible but may take 6–12 months to resolve.