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Adding Extensions


In these days of sky high residential property prices, extending your home seems to make a lot of sense. You can add space, add value, improve the quality of your living environment and retain the connections of where you live. Extensions can range from the mundane to the sublime and whilst budget has a significant bearing on what you can get, so equally does your and your architect/designer’s imagination. The following is some (hopefully) useful guidance on the journey of procuring the extension of your dreams.

General Design Principles
Most extensions are to the rear of a property, where that all-important connection with the garden can be emphasised. Something to consider is the extent to which any existing rear room(s) may become "light locked" or less desirable spaces as a consequence of having an extension built. There are two main factors that affect the above: one is natural light in, the other is views out.

Light in - This can be achieved via rooflights within the extension roof, throwing light into the existing rooms (see below). An advantage of pitched extension roofs is that rooflights (Velux etc.) within them will angle light deeper into the space than flat roof rooflights. Another factor governing the amount of light reaching back into existing rear rooms is the height of any structural opening formed between the existing rear rooms and the extension. The higher the structural opening i.e. steel beam height, the more light will flood back in - every inch really counts here. And whilst forming structural openings is generally quite expensive work, it will be money very well spent. Similarly, try and minimise the size of side piers (projecting masonry that support steel beam/s), particularly if the intention is for new and old spaces to flow into each other and be perceived as one space. Sometimes steel columns or loadbearing brickwork can be buried in the main walls, thus eliminating piers completely. Speak with a structural engineer about this (see Ascent Architecture example below). Alternatively, think about ways of integrating the piers into the design e.g. as a stopped end to a run of kitchen worktops.

Views out - These can be achieved with large windows or patio/bi-fold doors within the extension rear wall. Often, moving a kitchen back into original rear spaces, with a dining area positioned within the extension space works well.

Formal planning approval may be required. For domestic extensions this is normally covered under a Householder Application to the local authority. There is however something known as Permitted Development (PD), which are prescribed ways and extents to which you can extend your home without the need for planning approval.
For example you can build a single storey rear extension up to 3m deep to a terrace/end of terrace house or 4m to a detached house. And (until 30th May 2019) these distances are doubled to 6m and 8m respectively, but subject to something called Prior Notification (also known as Prior Approval or The Neighbour Consultation Scheme) whereby notification of your intention to build must be formally notified to the local authority.
A single storey side extension can also be built up to half the width of the existing house. Likewise rear dormers of 40 or 50m3 depending on the house type.
As you might imagine, with PD there are strict caveats regarding heights; whether you are in a conservation area etc. for which you should consult a professional architect/designer.
For an overview of Permitted Development relating to domestic extensions see here:
For more detailed guidance see here:

If you remain unsure as to whether your proposed extension falls within Permitted Development you can apply for a Lawful Development Certificate from your local authority to obtain unequivocal confirmation. Alternatively, if you believe your proposed extension falls outside of Permitted Development and requires a Householder application, but you think the proposal may be contentious, you can make what’s known as a Pre-application Enquiry to your local authority. For all planning-related applications to a local authority set fees are chargeable.

In all cases seek professional legal advice from an architect/designer and/or contact your local authority planning department.

Construction Options
There are numerous options for each element of an extension:

  • Walls - These can be masonry cavity; timber framed; a mixture of timber frame and masonry; solid masonry walls with internal wall insulation (IWI); solid masonry walls with external wall insulation (EWI). With IWI and EWI proceed with caution and seek expert advice owing to the risks of condensation occurring within the thickness of the wall.  
  • Floors - These can be solid ground bearing concrete with insulation either below or above the slab; suspended timber with insulation between the joists. With the latter ensure adequate cross ventilation of the sub floor void and of the sub floor void of any rooms blocked off by the extension.
  • Roof - These can be pitched with either insulation between & under, or above the joists; flat with insulation between and under or above the joists. Other than with insulation above the joists (known as a warm roof), ensure adequate ventilation across the roof to avoid condensation build up.

Building Regulations
The other necessary application to the local authority (LA) is a Building Regulations application. This is concerned with compliance with the Building Regulations which are divided into Parts A to Q. These parts cover aspects ranging from structure and insulation, through to ventilation and drainage. For an application submission, drawings in a larger scale (than for the planning application)  are required with details and notes showing compliance with the regulations. 

A Building Regulations application takes 5 weeks to be determined by the local authority Building Control department. For domestic extensions LA fees are normally fixed, dependent on the type and area of the extension. A portion of the fee is chargeable for checking the plans, with the remainder due for the four or five site visits required by Building Control officers during the construction phase.

The involvement of a structural engineer is normally required as part of a Building Regulations application, to undertake calculations and provide drawings as necessary to confirm formation of structural openings, foundation sizes etc. Their output should ideally be submitted to the LA within the 5 week determination period.
This same page on the Planning Portal gives a great overview of the issues to be considered (scroll down to Building Regulations).

Other Legalities
Law requires compliance with The Party Wall etc. Act 1996 wherever building across or close to the party wall shared with neighbours. The simplest means of compliance is where both parties discuss and agree to the proposals and sign an agreement. Simple example agreement letters can be found in the government's helpful Party Wall Act guidance document: If agreement cannot be made by both parties then you will need to appoint a party wall surveyor at your expense, and if agreement still cannot be made between the neighbour and your party wall surveyor, then a second party wall surveyor will need to be appointed to represent the neighbour; also at your expense!

Friendly, informal discussions with neighbours at the earliest possible stage are often the most effective way of smoothing the passage of compliance with the Act. Should access be required to a neighbour's land to access parts of your property, there is the Access to Neighbouring Land Act 1992. This would be relevant in the example of an inaccessible side wall of a Victorian terraced house's part-width rear addition. In particular I'm thinking about the External Wall Insulation (EWI). The Act serves to allow access for 'the maintenance, repair or renewal of any part of a building or other structure'. Whether EWI would fall under this description needs to be confirmed. There is also the question of whether the thickness of it is effectively "stealing" the neighbour's land. Should this particular scenario apply to you, you should consult an appropriate legal professional.

Consequential Improvements
The process of building an extension could be an ideal opportunity to upgrade the rest of your home. For example:

  • Insulating an extension with EWI could be an ideal opportunity to do the same to the remainder of the house.
  • Building an extension (possibly with underfloor heating) may be an ideal opportunity, or even necessity, to replace an old boiler with a new more efficient one.
  • Having work carried out to the main house at the same time as constructing an extension will be cheaper than having it carried out independently. Similarly you will only need to live through the disruption of having work undertake once.
  • The improved thermal comfort of your new extension might highlight the inefficiencies elsewhere in the house e.g. cold and draughty sliding sash or hinged casement timber windows. Such windows can be draught proofed and often upgraded with modern proprietary slim double glazed units. Uninsulated ground floor suspended timber floors can be insulated between the joists etc.

Building an extension can be an opportunity to create more and better quality space, as well as improve the comfort levels within your home and as a consequence reduce heating bills. It can also serve as an ideal opportunity to undertake improvements to the rest of your home. The process can be creative, educational and hugely satisfying as well as times challenging and disruptive. But isn’t this always the case for worthwhile things in life?

This page was written by architect Steve Mardall of Ascent Architecture


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