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Should You Build Out? Or Up? Which Extension Type Is Right For You?

Following the recent changes to the planning system, it’s now easier than ever to build different types of extensions under permitted development. Permitted development is a very specific set of rules and if your planned extension is covered by these rules, then you won’t need to apply for planning permission – it is given automatically.

You can build an extension under permitted development if:

For all extensions:

  • No more than half the area of land around the original house will be covered by extensions or other builds. This includes sheds and other outbuildings
  • No extension to be higher than the highest part of the roof
  • The extension does not come forward from the front of the house
  • The materials are similar to the appearance of the existing building

For side extensions:

  • Can only be a single storey
  • Cannot exceed four metres in height
  • Can only be up to half the width of the original house

For single storey rear extensions:

  • No more than eight metres* depth from the original rear of the house if it’s a detached property or more than six metres* for any other house
    *Larger single-storey rear extensions are subject to the Neighbour Consultation Scheme. This means that for a single-storey rear extension between 4-8 metres for a detached house or 3-6 metres for a semi-detached/terraced house, your Local Planning Authority will need to be informed of the proposed work via a ‘Prior Approval’ application and your extension is subject to approval.
  • Cannot exceed four metres in height

For extensions of more than one storey:

  • Extensions of more than one storey must not extend beyond the rear wall of the original house by more than three metres or be within seven metres of any boundary opposite the rear wall of the house

Building additional storeys*:

  • If your home was constructed between 1 July 1948 and 28 October 2018 and hasn’t already been extended upwards, then you may be able to add one or two additional storeys without needing planning permission. You can add up to two additional storeys if the existing house consists of two or more storeys. If the existing house consists of one single storey or is a bungalow then you can add one additional storey.
    *Building additional storeys is subject to prior approval – you will need to inform your Local Planning Authority of the proposed work and the extension will be subject to the Neighbour Consultation Scheme. If your neighbours raise any concerns or objections, the local authority will decide if their objections are valid and whether your plans can go ahead.


You probably don’t have permitted development rights in a conservation area, on a listed building or in an area of outstanding natural beauty. Check with your local planning authority if you’re not sure whether or not you have any planning restrictions in place.

(For full details, please see or contact Yoop Architects for further clarification/advice)

Sometimes there may be multiple ways to achieve your objective - Yoop Architects can take a measured survey of your property and create sketches to show sometimes five or six options to achieve your desired result.

So what extension type is right for you?

The two most important factors when considering what extension type you’ll need are functionality and budget.


As with all home improvement projects, the first step should be to consider what you need from your home. Start by thinking about how you currently use your home and the pros/cons of the existing space:

  • Which spaces work well in your home and why?
    Think about which rooms you currently enjoy spending time in and what it is about those rooms that you like
  • Which spaces in your home don’t work well and why?
    Are they too small or cramped? Overcrowded? Too dark? Too noisy?
  • What do you want to achieve?
    A home office? An extra bathroom? A family dining room or bigger living area? A larger kitchen? An area to host dinner parties that overlooks the garden? Don’t worry about working out what will go where at this stage, focus more on how you would like to live in your ideal home
  • Who are you building for and how long will the changes remain relevant?
    How much time, money and energy do you want to invest in a home office if it’s a temporary situation? Is the family unit expected to remain the same for some time or do you have grown children that may be leaving home soon? Do your plans include flexibility for the future or are you just thinking about how you’re living in your home right now?
  • What are your non-negotiables?
    Are you willing to knock walls down to create your ideal living space? If a rear extension is the best solution, how much garden space are you prepared to lose? What are your priorities?


How much you’re looking to invest will have a big impact on the options available to you. The cost of a house extension will vary considerably depending on the shape, size and function of the new space. If you’re on a tight budget, consider whether converting is a better option for you – you don’t necessarily need to build an extension to create more space in your home.

FREE GUIDE: Category: Home Extensions
FREE: Home Extension Cost Guide

Inside this easy to ready free guide, you will learn how much extensions, loft conversions and other renovations to your home are typically priced.

Conversion Build Costs

Loft Conversion

(Best roof type for the loft extension is a gabled roof (2 sides slope) rather than a hipped roof (all 4 sides slope))

Types of Loft Conversions

  • Room Loft Conversion
    Budget from £15,000

  • Dormer Loft Extension
    Budget from £20,000 or from £35,000 with the inclusion of an en suite

  • ‘Raising the Roof’ or Changing the Roof Structure
    Budget from £40,000

  • Hip to Gable Roof Extension
    Budget from £25,000

Converting a Garage/Basement

Turning an existing basement/garage into a usable living space costs from £700 per square meter for the build costs

Extension Build Costs

The main governing factor of the cost of an extension is square footage or square meterage – how big it is. So, the bigger it is, the more it’s going to cost. Typically if you worked out a square meter at £1500, that’s a good base price for basic quality; £1900 per square meter for good quality and £2200 per square meter for excellent quality.

Rear/Side Extension (Single Storey)

For an average single storey extension of 20 square metres, the build cost would be from £30,000 for basic quality, from £38,000 for good quality and from £44,000 for excellent quality.

Double Rear Extension (Two Storey)

Budget from £53,000

Second-Floor Addition

If the ground floor’s existing foundations can take the load, then you may have the option to build a second-floor addition on top of an existing room. This is much more cost-effective than building a new extension but will need planning permission if the addition is at the front of the property (i.e. above a garage)

Budget from £15,000

Additional Storey

Build costs start from £2,000 per square meter so for an average additional storey extension of 100 square meters, the build cost would be from £200,000. Another £7,500 to £12,000 worth of work needs to be added for lower-ground floor works like stair access and plumbing.


Additional costs that you may need to consider include:

  • Permission fees
    You’ll need to submit a ‘Prior Approval Application’ if you’re planning to build a larger single-storey rear extension or an additional storey. If your renovation work or extension comes under permitted development and you don’t require prior approval, then you don’t need to make an application for planning permission – you can just start building. However, we recommend that you apply for a lawful development certificate to prove that the work is covered by permitted development.
  • Professional fees (If you need an architect/structural engineer/project management)
  • Reports about your property (Tree/Flood Risk/Ecology etc)
  • Building Regulations Approval
    (You don’t need to get approval yourself if you use someone registered with a competent person scheme.)
  • Party Wall Agreement
    (Covers shared walls/structures and protects both yourself and your neighbours whilst work is being carried out)
  • Build Over Agreement
    (If you’ve got a shared drain running across the back of your garden and you’re going to build over it or within three meters proximity)
  • Insurance
    If you are carrying out conversion/extension works you should arrange specialist insurance to cover the new works and the existing structure as most home insurers won’t cover works if contracts exceed £50,000. For expert advise on this and any other insurance exposures, please contact Gary Philip at GPS Insurance Brokers on 020 8207 7385. GPS Insurance 

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If after taking into consideration what you need from your home and how much you want to spend, you’re still not sure what extension type would be right for you, contact an architect. There are two main groups of architects – those who do the Design & Planning of buildings and those who do the Delivery of buildings (some architects do both).

A ‘Design & Planning’ architect will be able to look at what you need from your home, the existing structure in place, your budget, local building rules and regulations etc and put together a variety of options that achieve your desired result.

At Yoop Architects, we achieve this by taking a measured survey of your property, which captures all the information. Then we draw it all up in 3D and create sketches to show sometimes five or six options so the process is very visual and easy to see and imagine what the finished result would be like. 

If you have any questions about renovating, planning an extension or just about creating your dream home in general, go ahead and book a free call below with Yoop.


Thinking of your dream home improvement or how to start your project? 
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Book a FREE 20 Minute Call with one of our home improvement Architects.

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Post Author: yoopadmin