How to: Do a rear addition

When home owners think about extending their living space, the typical 3 to 4m rear addition probably springs to mind. 1950s terraced houses (or similar) lend themselves to a rear addition as they usually have a flat rear wall and a decent sized garden, which gives ample scope to extend outwards without losing too much of the garden. The newly formed single story extension will probably feature a kitchen and a dining area which will benefit from natural light via a large roof window. 

Up until recently, a rear addition would usually feature folding doors opening up to the garden, however we are starting to see a reversal in this trend, with more clients favouring sliding or patio doors with an adjacent window. This arrangement provides access and views over the garden, whilst still maintaining wall surface on which to position worktops or non-fixed items of furniture. 


Rear additions typically cost around £3,000 per square meter for the shell and mid-range fittings. Costs will rise if high end designer kitchens are specified and/or specialist glazing packages are required. 


Owners of flats will need to apply for full home owners planning permission if they are building a rear addition. If they are a leaseholder they may also need to gain a license to alter from the landlord.

Owners of houses can extend their property under permitted development rights. Most home owners will be able to develop 3 meters beyond the rear wall of their property and up to 6 meters if they notify their Local Authority through the permitted development larger projects scheme. 

If the owner is extending under permitted development then they will be required to use materials similar to those in the existing property. This means that if they wanted to create a modern masterpiece, planning would need to be sought as the materials would be in stark contrast to the original dwelling. 

In applying for planning you will need to appoint a Surveyor, Architect or Architectural Designer to draw up the existing plans of your entire home, as well as the proposed plans for the rear addition. You should allow £1,500 to £3,000 for this service.


Most rear addition builds will take between 4 to 6 months to complete. The exact time will depend upon the complexity of the build and the access arrangements on site. 

If your property is a house you are likely to be able to build a 3 or 4 meter addition straight away under your permitted development rights. Larger additions may also be constructed under permitted development, though these require notice to your Local Authority. It is also worth considering gaining a certificate of lawful development if you do use your permitted development rights; this will assure a future purchaser that the extension is permissible.

If all goes as planned, some clients may be able to go from concept to a completed rear addition in 6 months, but it is best to allow 9 months to a year from initial concept to occupation.


The works will be taking place on the ground floor of your property and there is likely to be extensive disruption. This can be slightly mitigated if you have side access and a large property where a secondary living/kitchen space can be set up away from the works. In a smaller terraced home the works are likely to make the property difficult to live in, so the costs for temporary accommodation during the build should be factored into your budget.


Openings in your existing property will be required in order to use the new space in the rear addition. If you already have patio doorsets leading to the rear of your home then these openings may work with your new layout thus saving you the cost of any additional structural works. However, if you are looking to form a wide seamless flow from your existing home into the rear addition you will require a beam or steel 'goal post' to support this new opening.  

To meet building regulations you will require a structural engineer's design for this element, along with their supporting calculations. You should allow £500 to £1250 for their calculations plus  (depending upon complexity). 


In terraced dwellings the main sewer usually runs through the rear gardens. If you notice that the toilets in your house connect to a vertical pipe that disappears into the ground at the back of your home and that you have a man hole cover where you are thinking of building your extension, then this indicates that you will be building over a public sewer. 

This will require a 'build over' agreement application with your local water authority. For this application fee and supporting drawings allow £300 to £500.

The Party Wall etc. Act

You are likely to need to notify your neighbours under the Party Wall etc. Act. The Act gives you rights of access over their land which you will need in order to build your rear addition. If they have yet to complete an addition themselves, you may be able to agree to build the new side walls along the line of junction between your two properties. This will give you slightly more internal space and may allow you to share the cost of these walls. Depending upon your relationship with your neighbours you should allow £1,000 to £4,000 for this element. See the Blog on 'How to Save Money on Party Walls' for more detailed information.


Whilst Victorian L shaped properties do hugely benefit from a side addition, some ground floor properties with a fair sized garden may benefit from a rear extension instead. This is due to the ease of construction and cost. We would be happy to talk to you about these options to weigh up the best way forward for you and your property.