Residential design guidance: Roof extensions

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1 Residential design guidance: Roof extensions Introduction Conversions of roof-space to useable accommodation can be a relatively cheap and effective way to extend residential property. However, roof alterations can have a significant effect on the external appearance of individual properties because the roof is one of the most prominent parts of a building. In addition roof alterations can have a considerable cumulative impact on the character of a street or neighbourhood. This guidance promotes good practice and establishes clear guidance for the consideration of proposals involving dormer windows and other roof alterations. Some minor roof alterations and modest dormers do not require planning permission, and are known as permitted development. More information on permitted development is available by visiting Is a loft conversion a practical proposition? The practicalities of a loft-conversion depend on several factors. Using loft-space has implications for the thermal efficiency of a house and alternative means of insulation will have to be considered. Some properties simply do not lend themselves to such alterations. These include: Properties where the whole roof comprises modern trussed rafters Older properties structurally incapable of supporting the weight of another floor Houses with limited headroom in the loft-space, including those with a combination of shallow pitched roofs (below 30 degrees), narrow plan-depths and hipped roof forms In these instances, works are likely to be expensive, technically difficult and controversial in planning terms. It may be that a loft conversion is not a practical option and householders should give consideration to alternative forms of extension. Objective of Planning Policy In controlling residential roof extensions the Council, as planning authority, will seek to ensure that the design, materials and overall shape of the addition is sympathetic to the dwelling and in harmony with the surrounding area.

2 Principles The following principles will apply to the consideration of all proposals incorporating roof alterations: 1. Roof alterations should relate well to; (a) the dwelling on which they sit, and (b) the context of the street or immediate surroundings 2. Where dormer windows are permitted on front or other prominent roof-slopes they should be of traditional design 3. Where dormers are required for headroom or to provide additional floor-space they should be positioned on rear or less prominent roof-slopes 4. The privacy of neighbouring occupiers should be protected from undue overlooking These principles are explained in the following guidance. Detailed guidance 1(a) Roof alterations should relate well to the dwelling on which they sit The form, detailing and materials of a dormer should relate well to the roof form and detailing/materials of the main house. This is particularly important where the dormer would be visible from a public place. Roof pitches of gabled/hipped dormers should match those of the main roof. Window frame materials should reflect those of the main house. Glazing divisions and other detailing should reflect, but need not necessarily be identical to, those of the main house. Sympathetic form, detailing and materials Unsympathetic dormer Roofing materials should generally match those present in the main roof of the house. Exceptions include; (i) cases where the main roof is clad in large modern pantiles, where a change to a smaller flat profiled tile of the same colour and texture will produce neater detailing, (ii) instances where metal roofed flat dormers are appropriate, and (iii) eased out dormers where a material suited to a slacker pitch (eg. slate) may be appropriate, even if not present in the steeper pitch of the main roof. Dormers with modest, slender cheeks may be clad in lead, otherwise they should by rendered, weather-boarded or tile-hung depending on elevational treatment of main house.

3 Where a roof-slope is visible from a public place, the positioning of new dormer windows should be balanced and well related to the facade of the house. Dormers should follow the rhythm of the fenestration below, to reinforce symmetry. These are balanced This is unbalanced Dormers should be set well away from hips, verges, and other roof features such as gables, other dormers or chimneys, and should not break the original ridge or eaves lines (see advice on size, design and positioning of dormer windows in section 3). There are instances where dormers would not be appropriate on the front or other prominent roof slope of a property, regardless of design, because of the existing roof form of the house, for example, properties with roof pitches below 30 degrees where headroom requirements would produce a dormer of excessive bulk. 1(b) Roof alteration design should relate to the context of the street or immediate surroundings Roof alterations should not disrupt the perception of a dwelling s original roof form when viewed from a public place. Wholesale changes to the original roof form should be avoided. Replacement roofs that increase ridge height will normally be refused if the dwelling is attached or forms part of a composition of uniform detached houses.

4 Increasing roof pitch to enable accommodation in loft space can lead to exceptionally large roofs. In the countryside, this can increase the impact of the dwelling in the landscape contrary to rural protection policies. The character of the area immediately around a property should normally dictate the form of roofalterations. For example, changing a hipped roof to a gable should be avoided in areas where hipped roofs are part of the character of an area such as neighbourhoods or streets consisting of inter-war semi detached housing. This will be less of an issue in areas where hipped character has already been undermined by gabled additions to neighbouring dwellings or in other cases where widespread changes to roofs have taken place. There are instances where dormers would not be appropriate on the front or main elevation of a property, regardless of design, because of the character of the area. Some areas of semi-detached or terraced houses are characterised by long runs of visible roofs comprising smooth planes without dormers, houses and their roofs form part of a unified composition. In such areas, the introduction of front dormers would be uncharacteristic and visually disrupting. These areas include neighbourhoods of interwar semi-detached housing where front dormers would also affect the balance of individual pairs of semis, and tight groups of Edwardian/Victorian terraces. 2. Where dormer windows are permitted on front or other prominent roof-slopes they should be of traditional design Traditional dormer windows provide light rather than headroom or additional floor-space, they have modest proportions a good rule of thumb is the smaller the better! Traditional dormers are usually well contained within the bulk of the roof on which they sit and they are not intrusive on the roof-plane. The most common traditional forms are shown opposite:

5 Hipped dormer common on many house types from C17 to early C20 good if hipped roofs present in main house, detailing can be cumbersome so best avoided in pantile roofs. Gabled dormer perhaps most common, variants seen from C16 to present day. Cat-slide or eased out dormer most common on small cottages and terraces. Work best on houses with steeply pitched roofs. The slacker pitch of the eased out section may sometimes justify a change in roofing materials. Metal roofed flat dormer fairly unusual and best used sparingly. Top usually covered with lead, zinc or copper, often with standing seam detail. Can have a slight segmental curve. Where modest can have minimal disruption on plane of original roof. 3. Where dormers are required for headroom or to provide additional floor-space they should be positioned on rear or less prominent roof-slope. Dormers required for headroom or additional floor-space will inevitably be large, distracting features on a roof. As such they should not be placed on front or other prominent roof slopes. Guidance on modern box dormers: Box dormer guidance Box dormers have been developed in the latter half of the twentieth century as a way of maximising internal accommodation. Their modern form does not sit well on traditional dwellings, and they dominate roof-slopes making them unsuitable for front or other prominent roof-slopes. Box dormers will not be accepted on front or prominent side roof-slopes. Within glimpsed side or visible rear roof-slopes they should: (i) Be set well away from hips, verges, ridge and eaves as well as other roof features such as chimneys or parapetted party walls, (ii) Visibly grow from the roof and not dominate the original roof form. Where positioned within a rear roof-slope not visible from a public place dormers will be acceptable in principle but they must be designed to enable the retention of the dwelling s ridge tiles in their original position. Box dormers can dominate roofs Note see also general dormer guidance which follows

6 Summary of advice on the size, design and positioning of all dormer windows Front or prominent side Glimpsed side or visible rear Rear not visible from a public place Design Dormer may not be Dormer acceptable in principle Dormer acceptable in acceptable regardless of (subject to overlooking) principle (subject to 4) design (see 1a and b) Design important Design less crucial than on Design not so critical a front or prominent side Traditional form required Larger traditional styles Box dormer acceptable with form/detailing acceptable or modest box in principle sympathetic to that of dormers host dwelling Size Maximum width of 1.4m * Maximum width of 2m * No width restriction (see notes on positioning) Dormers should not occupy Dormers should not occupy more than one third of more than one half of the the width of the roof * width of the roof * Positioning No plane of a dormer No plane of a dormer Top of dormer must not should encroach within should encroach within break original ridge line metres of hip line or 0.6 metres of a hip line or ridge tiles to be retained verge ++ verge ++ in original position Dormer to be set 0.3m clear Dormer to be set 0.3m clear of any chimney, gable or of any chimney, gable or other roof feature ++ other roof feature ++ Set top of dormer at least Set top of dormer at least 0.3m beneath ridge m beneath ridge ++ Front of dormer to be set at Front of dormer to be set at least 0.6m from eaves least 0.6m from eaves (measured up roof-slope) (measured up roof-slope) New dormer to be set away from any existing dormer in the same roof plane by the width of the existing or new dormer (whichever is greater) Dormers should relate to fenestration in wall below * See illustration (i) opposite ++ See illustration (ii) opposite

7 (i) Size restrictions A = 2 metres max A = 1/2 B max Front or prominent side Glimpsed side or visible rear Nb. Width of hipped roof to be measured across roof from midway between ridge and eaves. (ii) Positioning limitations 4. Overlooking Big dormers with large expanses of glass give a greater perception of overlooking than modest traditional dormers. Front facing dormers usually address public space, such as a street, and do not therefore usually cause problems associated with overlooking. Rear facing dormers are usually orientated towards a dwelling s own private garden. Some degree of overlooking into adjoining dwellings is caused but this is usually indirect and is therefore tolerable. Privacy can however be an issue in high density developments where overlooking has otherwise been designed out. Side facing dormers will normally be refused if they are orientated directly towards the private patio space immediately behind adjoining dwellings.

8 Glossary Visible from a public place can be seen from any street, footpath, courtyard, square, waterway or park. Prominent roof slope the roof plane above a main public elevation (usually the front, often also the side) of a property, that part of the roof which can be readily seen from a public place rather than merely glimpsed. Less prominent roof slope a side facing roof plane that is seen only in glimpsed views from any public place, or almost any rear roof slope. Character of an area the qualities that affect our experience of a place. In combination, buildings and their component elements (forms, detailing, materials etc) can create strong character through uniformity or variety. The influence of the character of an area on the considerations of roof alteration proposals at a property is limited to that part of the street or place visible from the site. Exceptions This guidance does not apply to listed buildings or dwellings in conservations areas, where more rigorous case specific design control will be applied. Where area specific design guidance has been adopted, for example within Village Design Statements, that guidance should be read alongside this document and may take precedence over it. The advice contained in this guide is not intended to preclude bespoke, innovative architect designed solutions which may, on occasion, involve departing from some of the principles in this guide. Other guidance This guidance froms part of the Council s design advice on a range of extension types. More information can be found out at Building regulations approval Loft conversions and structural alterations to existing roofs require the benefit of building regulations approval. More advice on the regulations may be obtained from the Council s Building Control Section, tel: Council planning contacts General planning enquiries Conservation Officer (re: listed buildings) Planning Design Chelmsford City Council Civic Centre, Duke Street, Chelmsford, CM1 1JE planning.design@chelmsford.gov.uk Web : September 2013