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Thinking about adding an extension to your home?  This informative design guide will provide a useful starting point.  We have compiled this from our experience as architects working on residential extensions in Bath, Somerset, Wiltshire, Dorset and Gloucestershire, although the principles can be applied more generally across the country.  Additional considerations will apply if your house is listed, and you may also want to look at our companion guide dealing with listed building consent.



In the first instance, as your architect, we will help you consider how much space you need, what you need it for, and what other spaces (inside or outside) it needs to connect to.  What is the aspect of the proposed space: it is possible to take advantage of a particular view and how will the space be lit as the sun moves through the sky?  When adding the extension, what existing spaces within the house will be affected?  Many people now want more open-plan living and see an extension as a way of generating this in conjunction with existing rooms. Perhaps you will also be renovating or refurbishing other parts of your house.




Budget will always be an important factor when extending your home and we will help you understand the costs involved.  The size of your extension will have a significant impact on cost, but there are other considerations.  Two storey extensions will be more cost-efficient per floor area gained than single storey extensions.  Accommodation in a roof space will typically be more cost efficient than an area with conventional walls.  Basements or areas where the internal floor level is lower than the ground level will be expensive.  Large areas of glazing will be costly. 

It is important to consider not just the cost of building the extension, but also professional and council fees, any work that is required to the existing house (including renovation or refurbishment), landscaping and services.  If your extension includes a kitchen or bathroom, you will need to allow for the fitout of these areas, including the additional plumbing and electrical work required.



The majority of the residential extension projects that we work on require planning permission.  Occasionally work can be undertaken with Permitted Development Rights and a planning application is not then required.  In such circumstances, we advise our clients to obtain a certificate of lawful development, which when they come to sell the house, they can pass on to the buyer’s conveyancer.




One of the questions we are often asked is: how much can I extend my house?  This depends on a number of factors.  The further you move away from what is allowed under permitted development rights, the more of a challenge it becomes to secure planning permission.  The general planning principle is that an extension should be subservient to the main house.  It should not appear to dominate in volume, scale or height.  The Planners will also be interested in the impact that a proposed extension has on your neighbours.   If you live on a street with similar houses, then a good starting point is to see if other people have done the same thing that you want to do.


Front extensions

The front elevation, and any other street-facing elevations, carry a particular status.  This relates both to the outward appearance, and in many cases, the setback from the street itself (which might align with other neighbouring houses).  For this reason, it is rare to get permission for an extension on the front of house, beyond a porch or bay window.


Side extensions

A side extension invariably also has an impact on the streetscape.  Single or two storey extensions may be feasible here.  In our experience, widths of up to 65% of the original house are possible if handled carefully.  It will usually help if the extension is slightly set back from the front face of the main house.  In the case of an extension with the same number of storeys as the existing house, in order to make it appear subservient, it is often prudent to have lower eaves and / or a lower ridge. You may wish to retain access to the rear of your property which could impact on the width of a side extension on restricted plots.  Building within 1m of a boundary will also reduce the amount of windows that are permitted from a Building Regulations standpoint.

Rear extensions

There is generally more scope to extend at the rear of the property, and in the case of terraced houses, this is frequently the only possibility.  As with side extensions, maintaining an eaves and ridge below that of the main house is often good practice.  When extending to the rear, we will consider how the middle of the house will receive natural light.


Loft conversions

These can be a very efficient way to add more space to your house.  If dormers are required, then planning permission will need to be sought. If you plan to covert the loft of a listed building, then listed building consent will be required.  In all cases, care should be taken to ensure that Party Wall and Building Regulations issues have been considered.


Extensions in AONBs

In Areas of Outstanding Natural Beauty (AONBs), extensions are often considered by the Planners in terms of their volumetric increase to the original house.  In this context, ‘original’ refers to that which existed in 1948.  So if you are considering extending your house, then extensions, garages or outbuildings that have been added since 1948 will count against you.  There are no hard and fast rules, but from experience it easier to gain permission for extensions that amount to less than 50% of the volume of the original house.



The potential impact on your neighbours will be an important consideration for the Planners.  It is always good practice to keep your neighbours informed of your plans and to accommodate any reasonable suggestions that they may have. 


The Planners will consider overshadowing (the extent that an extension will cast a shadow over another property), overbearing (the perceived bulk of an extension when viewed from a neighbouring property) and loss of daylight / sunlight. They will also be interested in whether the privacy of your neighbours will be impacted, for example by the addition of a new window, balcony or terrace.  Side windows at ground floor are quite often acceptable, but side windows at first floor or above will typically need to include opaque glazing if they are near a boundary.


Rights to Light and Party Wall issues may also be important – see our Guide to the Process.






Generally, our approach is to work with the character of the existing house.  This need not mean replicating the details of the original house, and in many cases it is appropriate for the extension to feel more modest than the building to which it is attached.  In some scenarios, it may be appropriate to deploy a quite different but complementary character – in the case of an orangery for example.



When we consider the scale of a house, we are thinking about the size of the elements – typically the windows and doors – and also their subdivision.  In many cases, an extension should have windows and doors on the front elevation that are no larger than the existing house.  However, it is possible to have much larger feature openings at the rear of the house – bi-fold doors and large gable windows for example.  More glazing can also be achieved by adopting an orangery form.



More often than not, it makes sense to use the same materials for an extension as found on the existing house.  In some cases, an alternative, but harmonising material might be appropriate, to minimize the apparent mass of an extension for example.  More flexibility will exist at the rear of a house compared to an extension that can be seen from the street.  It may be appropriate for an extension that is designed to look more modest than the existing house (a garage for example) to use more humble materials.  If the house is in a Conservation Area, Area of Outstanding Natural Beauty, or World Heritage site (such as Bath for example) the council may provide guidelines on suitable materials.


We tend to specify traditional materials: stone, brick, render and timber cladding for walls; slate and clay tiles for roofs, and timber for doors and windows.  These materials weather gracefully, are durable and sustainable.



We believe that it is good to think about sustainability from the outset, rather than trying to tag on elements later in the design process.  Making sure the fabric is well insulated is critical; this will mean that the walls and roofs will be deeper than extensions built a decade ago.  Large windows in an extension will necessitate more insulation in the remaining fabric (the walls, roof and floor), and / or a better glazing specification. Substantial areas of south-facing glazing should be shaded to avoid overheating.    High quality double glazing or triple glazing will minimise heat loss.  Many of our clients are now asking for air source heat pumps, which are particularly  appropriate for underfloor heating in a well-insulated house.  It may also be a good time to think about installing solar panels.




It will be important for us to consider the external areas around a planned extension – paving, walls and fences, as well as trees and planting – and the way that these relate to the internal spaces and the external context.  The Planners are more likely to have concerns about elements that can be seen from the street, such as driveway paving and garden walls / railings.  In some cases, walls, fences or hedges may be desirable to achieve privacy. 



Our view is that trees and hedgerows should be maintained wherever possible.  However, there are situations where removing them is required.  In such circumstances, we encourage our clients to re-plant elsewhere on their plot.  On some sites, trees will be protected, either because the building is listed, lies within a Conservation Area, or because the tree is subject to a Tree Preservation Order (TPO). Specialist advice may be required from an arboricultural consultant in order to submit a report to the council.



Parking provision

For residential extension proposals, generally the Planners will be looking for the number of parking spaces to be maintained.  So for example, if you are converting a garage to habitable use, or building over a parking area, then the lost spaces will need to be allocated somewhere else on site.  The Planners will look at the number of bedrooms and the location of the house to determine how many spaces are required.  Rural locations will generally require more parking spaces than city-centre locations.  There may already be sufficient parking spaces to cover the required allocation, or it may be that the driveway area will need to be extended.


New vehicular access points

New vehicular access points will need to take visibility into account, for both oncoming vehicles and pedestrians (in either direction).  Where a wall needs to be demolished in order to create a new access point, we will consider whether it is protected – this may be because the building is listed, lies in a Conservation Area or is subject to Article 4 Directives (which is common throughout Bath for example).


Vehicle manoeuvrability

Existing vehicular manoeuvrability arrangements should be maintained.  If there is currently space on site for a car to turn and access the street facing forwards, then the Planners will be looking for this situation to remain.



If extending the total roof area of a house, then a soakaway will be required to dispose of the additional rainwater (assuming a dedicated rainwater sewer is not available).  Large areas of paving will need to be porous or be drained to a soakaway.  We will consider the impact that an extension will have on existing services such as soil downpipes and sewers.


Consideration will be given to any proposal that would impact on the habitat of a protected species.  Some geographic areas have special measures in place which provide additional protection. Trees and hedgerows may be providing a habitat for birds, bats and dormice.  Grassland that is being built on, or ponds / watercourses that are being disturbed may be providing a habitat for badgers, newts or slow worms. Existing roofs or derelict buildings may be providing a habitat for birds or bats (often a consideration when undertaking a barn conversion).  In some circumstances, care will need to be taken with lighting proposals that would have an impact on bats.  We will work with an ecologist to find a solution that allows a proposal to go ahead, and which maintains the habitat of the protected species through mitigation measures.



Following planning approval of your extension, work can begin on the more detailed aspects of the design.  These are described in our companion guide to the process.


Hopefully you will now have a better sense of the considerations when designing a house extension.  Please get in touch if you would like us to help you realise your vision.



DISCLAIMER. The information provided above is general in its nature and is based on our experience of previous projects.  It should not be considered to constitute professional advice for your particular project and should not be relied upon as such.



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