Extension: A Beginner’s Guide

Building an extension to a house is an increasingly popular choice for homeowners seeking to maximize the potential from their property space. As well as increasing the value of your home, building an extension can improve the space and layout of your home.

It’s an exciting idea but at the same time it’s also a big commitment, both financially and emotionally, and with this in mind it’s important that you get the design, build and budget right. Our guide to building an extension can help you, no matter the size or whereabouts you want to add the extension, there are a number of things to consider, from site access services to heating and electrics, as well as Party Wall agreements and the materials that you will need, all of these areas will affect how long your extension is going to take.

Are you building an extension for the first time? Check out our guide to planning and building the best space for your budget.


The price will depend on a number of factors, from the shape and size of the build, as well as the construction and the materials that you will use to build the extension. Costs in the UK will vary but it’s also important to remember that there can be material shortages as well as demand for trades, this will impact all costs.

For a straightforward extension we advise calculating around £1,000 to £2,000 per m2. Building a two storey extension can be a lot of extra building but it won’t cost much per square metre than it does to build a single storey extension.

Obviously there will be extra costs for the interior fixtures and fittings, but the extension itself will just require adding walls, floor joists, a roof and foundations, no matter how many storeys you’re building.


The cheapest way to build an extension is to keep the shape, style and design as simple as possible, as well as keeping the roof flat, this will mean it has less complications, especially when considering more obscure roof structures.

You can also keep the costs down when it comes to building an extension, especially when it comes to the building materials and the construction system that you have. Concrete floors and block work can also keep the budget down and be a more friendly option, especially in comparison to an oak or timber frame. Concrete blockwork is available and most builders will know how to use the material.


House extensions don’t build themselves overnight, even Planning Application and Building Regulations can take months alone. In order to stay sane during the period you must be patient, as it may cause a sudden disruption to your normal life.

You may also want to think about what time of year, factor in things such as family events, birthdays, holidays which you want full use of your house without builders being in.


There are a number of modern methods that self builders use when it comes to building, ways that could potentially make building the extension a bit easier and quicker.

Beam & block floor

This is also known as ‘suspended concrete’, the floor has an advantage of being less prone to structural problems that can be caused by ground movement. Beam and block floors are better for large extensions where a house has a suspended timber floor than may need ventilation.

Liquid screed

Liquid screeds are quicker to install over sand and cement screeds, they won’t be cost effective for a small extension, so if you’re using UFH, liquid screeds will work well.


They can hold much heavier loads over conventional timber joists of the equivalent weight.

Monocouche render

Monocouche render can be prayed and pumped into exterior walls, this will reduce the needs for scaffolding and site costs, it can also be low maintenance, durable and they come in a range of colours.

Prefabricated Roof Trusses

When extensions need to match existing roofs and walls, the requirement for customisation tends to favour more traditional cut timbers. So if you’re building a large rectangular extension then prefabricated roof trusses can save you time and labour.


For some property alterations such as loft conversions and single storey extensions, you may not need planning permission. It’s best to get in contact with an architect to double check whether or not you may need to send a Planning Application to your council.


You may find that you’re able to build an extension under Permitted Development, this means that you won’t have to go down the formal planning route. Under Permitted Development, certain projects can be carried out including:

  • You can extend a detached property by around 8m to the rear, if it’s a single storey extension (6m for a semi or a terraced house), or by 3m if it’s a double.
  • Single storey extensions can’t be higher than 4m on the ridge and the eaves, ridge heights of any extension can’t be higher than the existing property.
  • Two-storey extensions must not be closer than 7m to the rear boundary.
  • Side extensions can only be single storey with a maximum height of 4m and a width no more than half of the original building.
  • Any new extension should be built in the same or a similar type of material to the dwelling.
  • Extensions must not go forward if the building line of the originally dwelling
  • In designated areas like places of natural beauty, conservation areas, side extensions need planning permission as well as rear extensions that must be single story.
  • An extension must not result in more than half of the garden being covered.

In August 2020 the rules changed meaning that two storey extensions on detached, semi detached and terraced houses can be fast tracked in the process with the local authority. There are some restrictions involved in this, this covers:

  • Once the building is complete, it must not be more than 18m high.
  • The floor to ceiling height of any additional storey must not be more than 3m in height or higher than the floor to ceiling height of existing storeys.
  • The overall height of the extension, including the roof, must not be more than 7m high.

Should you be planning on building a large extension then you will more than likely require planning permission and you will need to submit an application, you can enlist the services of a planning consultant to help you if necessary. It’s important to engage with your local authority early on and research local planning policies to know what it’s like to get approved before you submit an application. You can also submit an application online.


Many people design their own extensions, using structural engineers for advice. Others use a design and build company who can take care of their whole project.

You can find so much inspiration online for your extension. Perfect places to find different styles of extensions are on websites such as Houzz, or if the architect you’re using has a portfolio, you can try checking it out for ideas.


It’s not just because we’re architects that we say you should use a professional.

Big projects such as loft conversions and extensions are not simple DIY jobs and we’ve all seen DIY jobs go wrong on TV! You will need the services of an expert if you are planning on making major alterations to your home. In fact- part of the Planning Applications you will need proposed drawings from a qualified architect and other consultants such as structural engineers.


Do you share a party wall with a neighbour and could any of your work damage their property? If your extension involves building or digging foundations within 3m of the boundary, party wall or party wall struc­ture, or digging foun­dations within 6m of a boundary, the work will require you to comply with the Party Wall Act. In these cases you may need a surveyor to act on your behalf.

There’s also the social aspect of things – your plans could potentially ruin their way of life during the construction period, so it’s important that you let them know of your intentions right from the start.


As we said earlier, it’s good to plan well in advance. It may be a good idea to set yourself a timetable for completion. There is a chance that it will over-run, but if you lay down the ground rules from the beginning you may avoid living on a building site for months on end. Don’t underestimate the amount of concrete planning building an extension needs.


This tends to be about balance, so if you’re likely to sell your home, you don’t want to price yourself out of not making any of the money back. When it comes to building an extension, not only will it add value in terms of more space, however it may not be as cost effective when it comes to selling it. By adding a bigger extension, this should increase the value of your home and it may cost more to build to remember to try not to spend more than you’re going to make back on it. There’s generally a ceiling price that you’ll want to stay within, so make sure that the numbers add up.

For more information on house extension services, get in touch with Detailed Planning today.

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