How to plan for a house extension

As of the middle of June we have officially started our building work! I have another post going through our plans but have put the original floor plan and our new floor plan above as a reminder. Basically, squaring off the back of our utility, extending out by 1.35m across the full width of the house, and knocking through the dining room, kitchen and utility. Since the work started I have had loads of messages asking ‘Where do I even start? Who do I contact first?’

We really felt like that at the beginning (in summer 2019) and so I thought that I would put together a post on the order that we did everything in. Different projects will need different plans, however what we’re doing is pretty standard and will be a good place for you to start if you’re currently pulling your hair out trying to make sense of it.

So here we go, if you’re just starting out planning an extension, here are the things that you need to consider and the people who can help you with it.


Now, you don’t necessarily need an architect to draw up professional drawings for a house extension. For projects which are pretty straight forward, which won’t require planning permission you might decide that you don’t want to include an architect. We knew that we wanted a fair bit of structural alterations and that we were going to have to go through planning permission and thought that it would be easier with an architect on board.

Personally, for a project like this, I would 100% recommend an architect. It was so valuable for us. I have written in a previous post about the different plans which our architect drew. We got quite a few different suggestions and space saving solutions which we just wouldn’t have come up with on our own. We also set out our budget right at the beginning, and our architect did a good job of giving us a rough idea of what we could do with our money. If you don’t have a clue about how much a build might cost this is really helpful to understand right at the beginning of the process. Our architect also sorted out our planning permission and structural engineer calculations making the whole process really easy on our side.

Like I say, if you already have an idea of the work that you want and it’s straight forward maybe you can skip this step, but for us it was totally worth the money!

** For a fixed price (including as many drawings as necessary) we paid £1100.

Planning permission

I have had so many people say ‘but if you’re only extending out by 1.35m why did you go through planning permission?’. Trust me, it’s not something we opted to do for the fun of it.

Here are the rules for permitted development for a single storey extension (taken from the planning portal):

  • Single-storey rear extensions cannot extend beyond the rear wall of the original house* by more than four metres if a detached house; or more than three metres for any other house.
  • Where not on Article 2(3) designated land* or a Site of Special Scientific Interest; and subject to ‘prior approval’, the limit for single-storey rear extensions is increased to eight metres if a detached house; or six metres for any other house.
  • Single-storey rear extensions cannot exceed four metres in height.

So, we aren’t breaching the 3 meters that we are allowed to extend by under permitted development, but we are breaching the four meters in height which we hadn’t appreciated when we bought the house. The height is measured from the lowest point of your plot of land. Out the back of our house there is quite a big drop, and the height is measured from the lowest point in our garden. For this reason we needed to go through planning permission. If you have an architect they should be able to help with your application, or you can apply through the planning portal yourself.

Because our architect sorted this out we found it really stress free and our planning was approved first time around!

**As a note, we think that we paid approximately £230 for our planning application.

Structural Calculations

When we were planning to do the extension, the most important thing that we wanted to know was how much will this cost and can we actually afford it. We wanted to jump straight in to getting quotes from building firms. The problem is that for a detailed and accurate quote for a project like this you will need structural calculations. To get structural calculations you need decent plans. So to get an accurate idea of what it is going to cost you, you’ll need to speak to a structural engineer. I mean, if you’re planning any work which will impact the structure/stability of your house you’re going to need an engineer on board anyway.

Over recent weeks I’ve had lots of questions about how we found an engineer and whether we had any recommendations for one. I hadn’t realised that they’re like gold dust at the minute! Fortunately, our architect worked with an engineer and sorted all of this out for us (another reason why I’d recommend working with an architect!). He sent his drawings over, and within a few weeks we had what we needed from the engineer.

The cost of a structural engineer will completely vary based on the type of project that you have. Ours, although pretty standard, was slightly more complicated as we’re essentially completely removing one corner of the house. We need a fair few RSJ’s for support. So as a bit of a guide, for what we’re doing, it cost us approximately £500 for structural calculations.


Once you’ve done all of the things I’ve already talked about you’re all set to go and get quotes from building firms. We looked for recommendations in all sorts of places such as local facebook pages, instagram accounts and from people we know. We sent out our architect drawings and structural calculations to 15 building firms, and 8 got back to us. We had all 8 around to look around the house and got quotes back from each of them. Quotes varied massively in price range (about 50K difference in quotes) and in how detailed they were.

I’ve listed the kind of things that we asked builders here:

  • What experience do you have of similar builds?
  • Can we get references from previous clients?
  • Is the price that you’ve given a quote or estimate?
  • What are the payment terms?
  • What exactly does the quote include?
  • How long will the work take?
  • Do you have liability insurance?
  • Do you think there will be any problems/difficult parts of the project and how will you deal with them?
  • Will you liaise with building regs/architects?
  • What is your availability?

So as always, this is just what we have done for our project and other projects may be different but hopefully it gives an idea of the type of things that need to be considered whilst planning a single storey extension to your home.

Any questions, drop me a message on Instagram!


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