• Draw Plans


Updated: Aug 28, 2020

If you haven’t built an extension before then it is crucial to the success of the project that you get a real grasp of all the costs before starting work.

At Draw Plans, we take several calls per day from homeowners thinking about building an extension and very few of them have a complete picture when it comes to the various costs relating to building an extension.

What follows is a summary of likely costs for building an extension.


Architects invest a lot of time and money in becoming an architect and so it is reasonable that they charge a fair amount for their skills. For a single storey extension you would be looking at a minimum of 6% of the build costs and for larger projects the range might be extended from 6% to 14% depending on the project, the time involved and the complexity of the design.

So, if you were building an extension, single storey, for around £60k then you would be looking at around £3.5k to £4k in architect fees.

Of course, if you were a bit clewed up you would realise that you don’t really need a RIBA or ARB architect for a such a small project. Neither do you need to pay anywhere near the amount an architect would charge if you were to instruct an Architectural Technician to create the plans for your extension.

Architects are highly skilled designers with a real focus on the design process whilst Architectural Technicians or Technologists are more focused on the science and the practicalities of the construction process. Having said that, all the technicians that I know have great eyes for the design and detail and are perfectly capable of designing a first-class extension for your home. The main difference is that they charge considerably less than an architect.

At Draw Plans, you should expect to pay an architectural technician around £1k plus vat for single storey extension plans in the London area.


Having extension design plans created for your extension is only the first step in the process. Now we need to consider the planning issues.

The good news is that most single storey extensions fall within the rules of permitted development. No doubt that when your designer visits he or she will discuss the planning issues and make a determination as to how you proceed. For example, you may need to:

  • Apply for Planning Permission

  • Apply for Prior Approval

  • Apply for a Certificate of Lawful Development

  • Seek Pre-Application Advice


A planning application will be required if are unable to satisfy the planning rules for permitted development or if you are unsure about whether your proposed extension meets the guidelines.

It is always better to be safe than sorry so if in doubt take the application route as you can’t go wrong.

Anyone can submit a planning application, but we advise that you have your designer submit the application because as the years go by the process has become more complicated. Here is an indication on what you may need to submit a planning application depending on where the property is located:

  • Existing Design Plans

  • Proposed Design Plans

  • Detailed Sections

  • Site Plan / Block Plan

  • Design & Access Statement

  • Water Plan

  • Flood Risk Assessment

  • Biodiversity Statement

If your property is in a conservation area or subject to article 4 planing restrictions, then you will have an even longer list of submissions to make a valid planning application.

At Draw Plans when we submit a planning application we monitor and liaise with the appointed planning officer to ensure that we get the desired result. If there is a problem or a design issue, we will be the first to know and will negotiate and work with the planners as necessary to get approval. Tweaking the plans, a bit and sending in revisions is considered normal for the planning process.

Draw Plans will be happy to submit your planning application for a house extension. Our cost for the service is £295 Plus Vat. You will also have the to pay the Local Council Planning Application Fee of £206 or £226 when submitted via the Planning Portal.


Anyone building an extension within 3 metres of another property or carrying out works to the boundary / party walls will require a party wall agreement. In effect, if you were building an extension, full width, on a terraced property you would require a party wall agreement with both neighbouring properties.

The most common type of party wall agreement is where you build your wall near to the boundary line but if space is really tight then you can look for a “Line of Junction Agreement” with the neighbouring property and if agreed you can build the wall directly on the boundary line where half of the wall is on your side and the other half is on the neighbours side.

Sorting out the Party Wall Agreements can be expensive as the average cost charged by local surveyors seems to be around £700 / £900 for a standard PWA with one property.

Draw Plans will be happy to sort out your PWA’s from £350 Plus Vat per dwelling so if you are building an extension or converting the loft and need PWA’s then get in touch.

Once other point is that most people don’t seem to know is that under the PWA legislation you are legally required to give your neighbour 2 months’ notice before starting works. And if you do start work without a party wall agreement your neighbour can have the work stopped.

Far better to talk to your neighbours early in the process and sort out the PWA’s as soon as possible.


Easily the number one question we get asked about house extensions because every homeowner wants to find out as soon as possible what their proposed extension is going to cost.

What you don’t want to do is to call the builders and ask them for a quote without design plans because this is simply a recipe for disaster. Even when you do have the plans done and you give them to the builder the price can vary by as much as 100%.

At Draw Plans, we do a lot of project management and we work with the builders all the time, so we are well informed as to how the builders work. We also know that many of the builders just chance their arm with big prices if they have a lot on whilst the reverse is true if they have little work in the pipeline.

The best way to get real quotes based on the actual work is to have your designer create a Specification & Schedule for your project. What this means is that your project is broken down into the various trades and various works and priced separately so that you have a price for every piece of work that is required. The builders hate pricing projects like this because if they over price on anything it is really easy to spot so it is a great tool to ensure that when you ask the builders to quote for the job you give them the plans and the schedule and that way everyone is quoting for the exact same job. You will be amazed at how much the price differs between builders for the same work.

Draw Plans offer all their clients a Detailed Schedule & Specification that is fully costed which allows them to create a reasonably accurate budget for their project before they call the builders. If the cost of the proposed work is over budget, we also work with our clients to show them the best way to reduce costs or to get more value.

It is essential that you know how much your extension is going to cost as early as possible to that you can make design changes or tweak the design plans as necessary to suit your budget.

As a guide, our cost to prepare a Fully Costed Schedule & Specification for a Single Storey Extension is £295 plus vat and in our view, it is money well spent.


Most house extensions will involve a bit of structural works to full integrate the new extension with the existing house. Usually, sections of the back or side wall will need to be removed to make a modern open plan kitchen extension. This means that a structural design has to be created for the openings along with structural calculations to prove that the design works.

The more walls you take out the more complex the structural design can be. For example, on very wide openings the engineer can specify a goalpost design of 3 steels or even a picture frame design of 4 steels to ensure that everything is supported properly. The more steel sections required the more costly it gets.