How much does a single storey extension cost?

So many of our clients ask us this question, I thought it would be helpful to give a guide for those of you wanting to know how much a house / home extension is likely to cost in Hertfordshire, Bedfordshire and Buckinghamshire before you proceed with the relevant costly and time consuming tasks such as planning permission, structural drawings and calculations by a structural engineer, a full set of plans and spec either by a design consultant such as ourselves or from a local Architect for more complicated projects. This article should leave you well enough informed to decide if an extension is right for you.

You may have thought about up-sizing to a bigger house as your family grows but you are put off by the cost of buying a new house or simply want to add some extra space to improve your existing home, an extension is an easy way to gain additional space whilst simultaneously increasing the value of your property.

Let’s have a look at the cost of selling a £650,000 family home to up-size to a larger house.

Re-mortgaging – £1,500
Agents fees @ 1.3% = £8,450
Conveyancing – £2,500
EPC – £120
Removals – £1,200
Stamp duty on purchase of new property (Assuming price of £700,000) – £25,000

Total Cost to Move = £38,000

With your average single storey extension costing between £35,000 – £60,000 could save all the hassle of selling your house and just extend and refurbish your existing property instead.

To have a basic extension would cost around £1,600 – £2,100 Per m2 in 2019, so a 5m x 4m single storey extension reputable builder would charge around £40,000. Any builder carry out this sort of work would be VAT registered which would add 20% to the builder’s price. So, the overall cost for a simple single storey extension would be around £48,000 in total.

For double storey extension, the cost increases by approximately 50% so prices range between £2,400 – £3,150/m2. A two-storey extension with the same footprint of 20m2 (40m2 over both floors) would cost between £60,000 – £72,000 including the VAT.

For more complicated extensions, an architect and structural engineer will be required adding a further 11-13% on top on the Build cost. So a double storey extension adding 40m2 designed by an architect would cost around £67,800 or £81,360 including VAT

Bear in mind that you are likely to add between £2,000 – £3,000 per m2 to the value of your property when you extend so in the long term, the extension will have paid for itself plus the additional attraction of having a modern open plan living space which would appeal to potential buyers in the future.

Many extensions nowadays fall under permitted development which make then easier to plan and would generally save £1000 – £2000 on the planning application, having full plans drawn and amended etc.

The Planning Portal states:
“An extension or addition to your house is considered to be permitted development, not requiring an application for planning permission, subject to the following limits and conditions:

  • No more than half the area of land around the “original house”* would be covered by additions or other buildings.
  • No extension forward of the principal elevation or side elevation fronting a highway.
  • No extension to be higher than the highest part of the roof.
  • Single-storey rear extension must not extend beyond the rear wall of the original house* by more than three metres if an attached house or by four metres if a detached house.
    In addition, outside Article 2(3) designated land* and Sites of Special Scientific Interest the limit is increased to 6m if an attached house and 8m if a detached house until 30 May 2019.
  • These increased limits (between 3m and 6m and between 4m and 8m respectively) are subject to the prior notification of the proposal to the Local Planning Authority and the implementation of a neighbour consultation scheme. If objections are received, the proposal might not be allowed.
  • Maximum height of a single-storey rear extension of four metres.
  • Extensions of more than one storey must not extend beyond the rear wall of the original house* by more than three metres.
  • Maximum eaves height of an extension within two metres of the boundary of three metres.
  • Maximum eaves and ridge height of extension no higher than existing house.
  • Side extensions to be single storey with maximum height of four metres and width no more than half that of the original house.
  • Two-storey extensions no closer than seven metres to rear boundary.
  • Roof pitch of extensions higher than one storey to match existing house.
  • Materials to be similar in appearance to the existing house.
  • No verandas, balconies or raised platforms.
  • Upper-floor, side-facing windows to be obscure-glazed; any opening to be 1.7m above the floor.
  • On designated land* no permitted development for rear extensions of more than one storey.
  • On designated land, no cladding of the exterior.
  • On designated land, no side extensions.


* The term “original house” means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date). Although you may not have built an extension to the house, a previous owner may have done so.
* Designated land includes conservation areas, national parks and the Broads, Areas of Outstanding Natural Beauty, and World Heritage Sites.”
Additionally, under the “Neighbour Consultation Scheme” you can increase the size of your single storey extension up to 8m for a detached house and 6m for all other houses.
With the information stated above, you can easily work out how much your extension is likely to cost and you are now one step further to making an informed decision that will greatly improve the space and feel of your home.
If live in St Albans, Harpenden, Berkhamsted, Rickmansworth, Chorleywood, Beaconsfield, Chesham, Amersham, Hemel Hempstead or Wheathampstead or anywhere in Hertfordshire, Bedfordshire or Buckinghamshre and you would like to have a free consultation from one of our estimators, please give us a call on 01442 773 119 , we are ready to help you with your project.

Written By: Matthew Britz
Company: Berg Contracts Ltd © 2019