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London House Extension

Building an extension for your property: how much would it cost?

Does the property feel  more and more cramped? Is the household getting bigger and will be soon in dire need of more space? Or perhaps, thinking of getting that office or playroom room you always wanted? There are thousands of dreams and reasons as to why build up an extension, too many to list. But it is clear, the we all want to make the best of our living space. And an extension may be just what you need.
Our team, at AFL Construction, have had the pleasure to make those dreams come true. Through our experience in building extensions for our clients, we have picked up a few things along the way that might help you into finding your reason.
One of the first question that always comes to mind is: How much would it cost to build an extension?
Do keep in mind that every property is unique, therefore it would be complicated to give a figure that would cover every type of residence. But after years of estimating extension projects, we can say that a typical terraced extension shell in London would cost  £35k upwards (subject to VAT).
For this brand new home space, the potential additions (assuming basic to mid range) and inclusive of building works and supplies would be:
  • Bi-fold doors £2k to £5k
  • Kitchen £3k to £15k
  • Bathroom £3k to £7k
  • Toilet or Utility room £1.5k to £3k
  • Flooring and skirting £3-5k
  • Lighting and electrics £1-7k
  • Roof-lights £1-3k
  • Alterations to the existing property are subject to scope and size. An average of £700 to £1500 per m2
  •  Landscape and external works are also subject to scope.
The figures above would cover the building cost of the project. However before building works start there are some pre contruction costs that you may incur:
  • Planning permission application,
  • Party wall agreement
  • Structural engineer fees
  •  Architect fees to for the design consultations before the works. You might also have the Architect to advice during the build.
  • Planning and construction drawings, and structural design. Which would altogether come to around £1,5 to £5k.
All in all, these figures are an approximate and we would strongly advice you to get a specific estimate of your project.
On another note, we would be happy to welcome you to work these numbers with our team at AFL Construction.
In order for us to help provide an accurate quote, we would need the drawings, scope of works and the schedule of the fittings.
We will come back to this later, as we would like to tell you first about the legal obligations entailing your future construction.

Extending your property: what are the legal hurdles?


Before starting to build your new space, you will need to have a planning permission, unless your property falls under the permitted development category.
Permitted development works are limited to certain heights, sizes and placements (e.g one-storey height limit for extensions). Generally speaking, building a larger project requires permission subject to the neighbour consultation scheme and others; and you will be required to attach the planning drawings along with your application.
To learn in detail about these permissions do check the Planning Portal ( https://www.planningportal.co.uk/info/200130/common_projects/17/extensions)  and the Government Website ( https://www.gov.uk/planning-permission-england-wales)
Informing your neighbours and getting their consent is essential before the project can carry on.
Should you be unable or find it complicated to discuss it with them due a variety of reasons, you may have to employ a part wall surveyor. The surveyor will take care of the agreement on your behalf but in return you will have to bear the costs and the process would take approximately 6 weeks.
It is paramount to notify Building Control ( https://www.planningportal.co.uk/info/200128/building_control) and pay their fee 48 hours prior to the commencement of the works.
Building Control is an entity within your local authorities that ensures the works comply with building regulations. Some private companies are able to provide the service as well.
Regulations will describe the acceptable standards for structure, insulation, fire safety, ventilation and other aspects of the building. Approved officiating inspector usually call on site at certain stages to perform various checks.The BCO will sign off an Issues Compliance Certificate at the end of the project when all regulations are met.
A personal advice from our team's experience is that you should see to met the BCO at least a week before the works kick off. This is in order to present the project and discuss any regulations that may affect the scope of works.
Having a well defined framework of the works will help you calculate an accurate overall cost and save you from unexpected variables during construction.

It's time to start building! How do we stay on time and on budget?

The client is keen to start and the builder wants to move in and get that job done. But before we start it is well worth checking that we have everything we need for the journey ahead.

  1. Scope of Works - A well defined scope of works does not necessarily have to be multiple pages long. Actually, we prefer it short and concise. It needs to contain the level detail that would allow us to give you a fixed quote.
  2. Construction Drawings - Not all drawings are the same. There is a difference between planning drawings and construction drawings. And these construction drawings together with the rest of the specifications will provide us with enough information for the other various trades involved with us in the project to complete their part of the job.
  3. Schedule of Fittings - This can be defined as a list of the fittings to be installed. Our advice is that you should do a detailed list and present it before the start. Otherwise, sudden additions or changes present mid-project would result in a number of variables and difficulties that may affect the process and the result.
  4. Procurement - This is simply for you to look and shop for special items and the like that are outside a general builders work scope.

It may seem like the builder should know the cost of an 'Edwardian slim glazed timber sash windows with lamb tongue profiles of various sizes', but truth is, in order to know the final and true current cost, we strongly advice you to get a quote from a specialist supplier for the specific products you'd like. It will mean less surprises on a budget front during the construction phase.


These suppliers might need to visit the site, and it will take time and also test your negotiation skills should you wish to handle it personally. However, you can also relay this task on the us or another specialist party, to manage things for you.


  1. Program - This is basically a schedule that will show when each task is started and when it will be completed. A program will help you gauge the progress and might also indicate sections in which the client's (your) input is required for a certain section.
  2. Contract - As with all written agreements, it is of vital importance to read and understand the contract that one will be entering. It will describe the relationship between builder, client (employer) and any administering parties such as architects, surveyors, etc. Do not hesitate to seek for help, should you not be sure about any part. Understanding the contract will help one to understand the differences between fixed costs and the ones that can vary. From our experience, trust us when we say that variations can be expensive to the client and a headache to the builder when trying to stay on course. It is critical that we all understand and stay on the same page.


If you have covered all of the above, congratulations! Now you are by far more prepared than 90% of our clients when they first turn to us with requests to start the works.


For anything else, do contact us - we are always happy to help!


On behalf of Andrew Draciovas (Managing Director) and the team, we hope to help you see your vision become reality!

Please check these examples of London house extension in our Projects gallery:

Back and side extension: Extension in Finchley

Side extension : Extension in Catford

Back extension : Extension in Peckham