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Updated: Nov 22, 2022

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.

Steel fabrication is also costly and necessary for certain projects. Every bit of steel bolted to another piece of steel needs to be fabricated for a precision fit onsite.

As a rough guide to structural engineering costs we generally allow for £300 / £400 per piece of steel required for the project. So, a simple of opening with one piece of steel can cost you £300 /£400 whilst a more complex goalpost design for an opening might be £900 /£1,200.

Draw Plans will be happy to quote for structural alterations and structural calcs for any build project. All we need is the design or building plans to prepare a quote.


When building an extension all work must be done inline with current building regulations, so you have to let building control know about your project.

There are two build routes that you can take with building control. Building Notice, where you submit a notice to building control a few days before you start work and a Full Plans Application where you submit a Building Control Application much the same as a planning application and get everything approved before you start your build.

We generally recommend the Full Plans Application for an extension as it is always better to have everything approved before you start any work. That way if there are any issues that involve additional costs you will know about it before the project starts rather then find out whilst doing the job.

Regardless of which build route you take, building control will require building regulations drawings for your extension. These drawings will show the construction details for your extension such as the foundations, the walls and how the roof is constructed. These drawings are also essential for the builder because they specify how the extension is to be built.

If there is structural works involved, you will also need to furnish building control with the structural design plans and structural calculations for approval.

Draw Plans will be happy to create Building Regulations Drawings for any type of build project. Just get in touch for a free consultation and quote.


Having a project manager manage your extension is a great idea because it may not cost you a penny and if you choose your project manager carefully then it could even save you money.

Most project managers will charge a percentage of build costs. At Draw Plans, we charge around 6% of net build costs which means that on a £50k extension you are looking at project manager fees of around £3k plus vat £3.6k.

If you instruct Draw Plans to project manage your build, then we will be involved in the builder negotiations and selection process and we will quite easily cut your build costs by 10% or more. In effect, the builder will end up paying for your project manager and it doesn’t really get better than that.

When it comes to big ticket items such as double-glazed doors and windows, flooring and kitchens and we will also show you how to get the best value for your money.

Draw Plans also offer a 10 Key Stage Project Management Service for £995 plus vat for house extension in the London area which will save you even more money. Just get in touch or browse our blog for more details.


Using use our scale of fees at Draw Plans we can give you a reasonable indication of what you will need and what they cost will be for building an extension.

  • Design Plans £ 795

  • Planning Application £ 580

  • Party Wall Agreements (2 Properties) £ 700

  • Building Regulations Drawings £ 795

  • Schedule & Specification Service £ 295

  • Structural Engineering Design & Calcs £ 350

  • Project Management 10 Key Stage PM £ 950

Now keep in mind that some house extensions will not require a planning application, and some will not require party wall agreements or a structural engineering design and calcs.

You can also submit your own planning application if you so desire and the schedule and project management services are optional.


Draw Plans are London based architectural designers and consultants that specialise in adding space and value at low cost. We provide Design & Building Plans for any type of House Extension or Loft Conversion within 7/10 days. Prices Start from £795 Plus Vat for a Single Storey House Extension.

Get in touch for an informal chat and a free quote for architectural services. 020 3921 0745

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