Well-designed house extensions can transform even the humblest of homes by maximizing the floor-plan and adding to the market value. Considering today’s house prices as well as tallying up the costs of moving (think solicitor’s fees, stamp duty, house movers and van hire), improving what you’ve already got can be a more appealing and affordable route to creating a living space that meets your family’s needs. Use this guide to help you maximise your property’s potential.

This article will be useful to anyone undertaking a house extension throughout England. If you are in Cambridge, St Albans or anywhere in East Anglia and London areas then we are local architects who can provide specific hands on help with your house extension. You can contact us here.


Modern house extension by Home Design Studio London house extensions guide HOUSE EXTENSIONS GUIDE Home Extension Plans London

Modern house extension


Your first steps should be to consider what you want to change about your property. This will form a good brief for your house extension project. At this stage it’s best not to focus too much on the specific ideas you think you want to pursue. It is better to say you need another bedroom and a bigger kitchen, rather than you want to extend the ground floor, on the right hand side, next to the current kitchen. When you can clearly state what you want your extension to give you, an architect can provide the professional advice you need to make it happen. The best layout for your extension may not be what you first thought! A typical brief for house extensions should include:


  • Allocated budget
  • Maximum budget with contingency

Problems with existing house, for example

  • Need an extra bedroom
  • Main bedroom needs an en suite
  • Kitchen is too small
  • The garage is largely under utilised
  • Need an independent annexe

Family needs, for example

  • Husband, wife, two children under 10
  • Grandparent lives with family
  • Lots of family and friends of all ages are regular visitors

Essential requirements

  • Need an independent annexe
  • More open space for entertaining

Future proofing

  • Allow for more independent space requirements for kids as they get older
  • More efficient boiler

Other important considerations

  • We like contemporary light spaces
  • We dislike beams
  • We live in a conservation area
  • Managing neighbors who have a party wall


Before you decide to extend, consider all the poorly used space in your home. For example, integrating old ineffectively used cupboards, large landing spaces or even dividing a large bedroom into two. You can consider converting existing space such as a loft conversion, basement or a seller or even making better use of a garage space. Taking this approach is usually cheaper than extending and will not sacrifice your garden space. Only consider a house extension when you have carefully evaluated all your options – preferably with an architect.


A seamless extension is when you extend your home so it looks like it was always built that way. This can be quite difficult to do as you will need to match materials and put old and new together. It is often a fine balance to source and use reclaimed materials together with new materials. An alternative option is to extend in a deliberately different style — for example, contrasting styles of period properties against very contemporary designs. Some example house extensions by Harvey Norman Architects are shown below.


  • Small gardens may suffer when reduced even further as a result of house extensions
  • Poor designs can deprive plants of much needed sunlight
  • Useless fragments of outside space, disjointed and cut off from the main garden
  • Lack of daylight and the feeling of space from a poor understanding of how light flows through your home and house extensions
  • Light that intrudes with usability of technology such as TV’s and computer screens
  • Passive solar gain that brings welcome heat in the winter but discomfort in the summer
  • Designs that boldly incorporate an altogether different style with great potential, but which produces a disjointed building.



Costs for house extensions vary widely depending on their size, style and specifications. You will really want someone to tell you specifically what you can get for your maximum budget. But this is very difficult to do, especially when your house extensions plans are nowhere near settled.

So at this stage, you will have to settle for ballpark figures. These figures can get more accurate the more specific your project gets. Generally, when using a main contractor for the works, you should expect to spend at least £1,500 per m2 upwards for a single-storey house extension project in standard brick and block.

You may be surprised to note that house extensions with multi-storey additions will come in slightly cheaper per m2 because they can offer the same amount of internal space but reduce your outlay on the most expensive parts of building projects — foundations and roof structure. If you are going for a higher spec, for example, opting for glazed walls, then prices per m2 will of course rise.



Not all house extensions needs local authority planning consent. You can save yourself the time and cost of making an application if your extension falls within permitted development (PD) rights. For example, a single-storey rear extension can be up to 3m deep for an attached house, or up to 4m deep for a detached property subject to a neighbour consultation scheme. In the UK, these limits have been stretched to 6m and 8m respectively until May 2016.

When you inform the council about a house extension with your permitted development rights, they must consult owners or occupiers of adjoining properties (minimum consultation period is for 21 days) — this is known as a neighbour consultation scheme. If none of the neighbors with adjoining properties object to your house extension then the council does not need to assess your proposed project. If some owners or occupiers of adjoining properties do object, then the council must assess the impact of the proposed house extension on all adjoining premises.

Ground-floor extensions to the rear of houses are rarely controversial when it comes to obtaining planning approval. This is because the main pre-occupation of planners is the impact the changes to your property will have on the surrounding neighbours. House extensions involving the first floor and above will therefore, attract more scrutiny. Despite this, permitted development rules allow you to add quite a bit to the original house before you need to make a planning application.

‘Designated land’ usually means the property is in a conservation area, an area of outstanding natural beauty, national park or similarly protected land. These permitted development rights do not allow alterations to properties on designated land or on listed buildings where consent is required for any kind of alterations.

If you do need to apply for planning approval for your house extension, note that how planners approach planning permission can vary between councils. So, find out what your local authorities standards are and how rigorously they apply them. An architect with local knowledge and contacts can be invaluable when considering general rules of thumb that most local authorities apply when assessing the application. Consider the following factors when drawing up a design and also consider 3D models to help all stakeholders really appreciate the potential of your house extension plans.


If the new addition gives you a prominent view directly into your neighbour’s garden, or into their windows that are close to the boundary, this will cause concern for planners. They will also assess whether significant daylight is lost to the neighbours’ ground-floor windows by a new section of first floor. In many cases, especially urban and suburban areas, an extension to one house is bound to have a noticeable effect on its immediate neighbours. The planners will have decide whether that effect is significant and ‘detrimental to the amenity’ of the neighbouring properties. Making this assessment can be quite subjective. In many cases neighbours object to anything that has any effect on their property at all, but a neighbour objecting is not grounds for a refusal of planning permission.

To help planners make consistent decisions, most local authorities use a combination of ‘rules of thumb’ and written policy. They often have written into their development policy set distances that must be achieved between the different types of elevation of houses. For example:

  • The fronts of houses may have to be at least 22m apart
  • The side of one house may have to have 7m between it and a rear elevation of another house
  • The minimum depth of a back garden can be set at around 10m
  • For first-floor house extensions to the rear of a house a 45 degree line is drawn on the plan from the edge of the neighbour’s closest ground floor window across the back of your garden — if the proposed extension crosses this line it will be at risk of being rejected.

These rules are mainly for new housing estates, but they will usually influence the assessment of applications for house extensions to existing properties. The problem with applying these general rules is that sometimes they are not appropriate or do not apply to a particular situation, for example, where there is a steep level change between the houses. It can sometimes be a challenge to get planning officers to accept variations like this and make a decision that goes against the grain.


When planners use the phrase ‘in keeping’, they are asking for the new addition to match not just the materials of the surrounding buildings, but also, in theory, their size, scale and proportion. If there is no dominant architectural style where your property is, it will be easier to gain consent for a design that is contemporary or ‘out of sync’.

Planners also prefer house extensions that are visible from the road to be ‘subsidiary’ to the existing house. In effect, the ridge height of the new part of the building is expected to be lower than the main ridge height. The latter can be difficult to achieve if a large addition of floor space is needed at first-floor level, and also illogical if the house can be made to look better by adding something of a similar size that matches in well with the original. Negotiation skills will be needed in these circumstances.

The ‘terrace effect’ is another phobia of planners! The thinking goes that if you have a typical suburban street, made up of rows of semi-detached houses and they all decide to add extensions close up to their side boundaries, the street will then look like a terrace of houses. This is considered a bad thing. The solution is to insist that any such extensions step the first floor back from the front face of the house. The street will then look like a row of terraced houses with the first floors close to the boundary stepped in. This is considered a good thing. This rule of thumb seems to be applied ruthlessly throughout the UK, the only difference being the distance that is considered an acceptable indent. Some councils insist on 2m, others will tolerate 1m. If it is far enough from the boundary, building the extension wall flush with the existing front wall may then be acceptable.

It is not unusual, particularly in older housing estates, for the front faces of the buildings to be in line with each other. Less planned roads also tend to have stretches of houses that are approximately aligned. This is referred to as the ‘building line’ and again, any new house extensions that are proposed nearer the road than their neighbours are considered to be a potential threat to the quality of life, order and harmony of the neighbourhood. If it can be shown that there is a building line along the road, and that your proposal breaches it, your application can be refused.


Whether or not you need to obtain permission for your house extension, you must consider all potential complications.

  • Permitted development rights are removed for listed buildings and houses in conservation areas
  • House extensions on listed buildings will always require separate listed building consent
  • If your project might affect the structural integrity of neighbouring properties (for example, if you’re excavating close to a boundary or underpinning shared foundations) then you’ll need to notify your neighbours of your intentions by serving a notice under the Party Wall Act.
  • As of April 2015 homeowners are now responsible for the safety on their building projects – big or small. All projects will therefore require a health and safety plan and you will be required to manage that plan.



Although all regulations apply to house extensions there are one or two aspects that are particularly worth noting.


In a normal two-storey house, the regulations stipulate that some of the upstairs windows should be wide enough and low enough for someone to climb out if there is a fire. These are called ’emergency egress windows’. One of these is required in all habitable rooms upstairs and also on the ground floor if the room does not open directly to the outside or to the hallway. A ‘habitable room’ is a bedroom or somewhere you are likely to be in for a while, but not rooms such as bathrooms or cupboards. Many standard windows comply with the minimum size requirements, but sometimes the lower window sill is an inconvenience, for example, in rooms-in-the-roof bungalows, where the roof pitch makes it quite hard to achieve. If a house has three floors, it has to have a protected fire-escape stairwell, removing the need for emergency egress windows for all rooms that lead off it.

Planning departments often limit the size and location of windows close to, and facing, the boundary. Building control officers also have restrictions. The latter is because, if a house catches fire, it can spread between properties through windows close to a shared boundary. So, the size and number of windows in an extension wall, close to a boundary, are restricted, particularly if the new wall is closer than 2m.

Another regulation that affects windows is the requirement to provide fresh air to a room. The regulations say that sufficient fresh air is available if the total area of openable window in a room is 20% the size of the floor area. There is a potential conflict between these two regulations — one limiting the size of window, the other requiring it to be larger. This can only be resolved by introducing mechanical ventilation.

A garage is considered a potential source of fire, because of the petrol in the fuel tanks of cars. There has to be at least thirty minutes of fire-protection between the garage and the rest of the house (that means it will take at least 30 minutes for the fire to burn through). The regulations also consider the risk of spilled petrol flowing under any doors into the house and require a step up from the garage to prevent this.


The rules controlling how much energy is needed to heat new additions to a house are far more demanding than the time when global warming was not recognised as a problem.

Regardless of how well or badly the existing house is insulated, the new roof, walls and floor must be up to current standards. If the area of glass minus the area of existing openings covered up by the new building work is greater than 25% of the floor area, then a calculation is necessary to show that the heat loss from the whole of the house, after it has been extended, complies with the regulations.

It follows from this that if you intend to increase the area of glass beyond the 25% figure, you will have to increase the insulation levels of the new walls, and possibly add insulation to the existing walls, to compensate for the heat lost through the glazing. The calculations that are needed are relatively complex and usually carried out by an expert.


House extensions can be a clever way to provide you with what your current property may lack. Carefully consider your needs and related design considerations. It is likely you will be able to use your permitted development (PD) rights and save yourself time and money by avoiding the need for local authority planning consent. However, there are stipulations and building regulations you will need to adhere to so do your research and get professional advice. Taking the time to this and working with an architect will help protect your investment and ensure all your hard work pays off.