Key considerations when designing a house extension

on Thu 5 Dec


What do I need to consider when designing a house extension?


Before you consider extending your house, it is important to ensure that all spaces within the existing house are being used most effectively and efficiently. You may find that with some moderate re-planning of existing rooms can avail the space you are seeking for a particular use either by making those existing spaces work harder in terms of 'adaptability' or reducing the area of largely unused space to create a new room. Adaptability refers to the ability of a space/room to be used in a various purposes without affecting its overall area or footprint of the house i.e. a bedroom could also be used as a study or a living room could also be used as a dining space. 

If there is no further space than can be utilised within the existing house or one particular room just needs to be larger then a house extension can be considered however only after carefully evaluating all known options in particular available funds. Extensions are expensive, the cost of which can often exceed the value of the newly extended space once complete in terms of its effective use and/or the market value of the property.

Thus, the planning of any extension must be based on a comprehensive thought process in terms of the principal reason(s) for the same. Such elements that should form part of the initial decision-making and reasoning for an extension are as follows:

  • what amount of 'currently available' funds (net savings that can be set aside at the earliest time and are at no risk of being needed to cover any other expenditures) can be allocated to house refurbishment works?
  • what portion of funds identified for refurbishment works are available to cover the 'construction' works after professional fees, local authority fees, insurances and VAT?
  • what amount as part of the above budget can be set aside as a 'contingency' to cover any unforeseen issues either in the design or construction stages?
Space priorities
  • what are the main spatial issues with the existing house?
  • which spaces are under utilised?
  • how much area is 'actually' needed to resolve the spatial issue(s)?
  • who (primarily) will need to use the newly developed space(s)?
  • is there available space within the existing house to deal with the spatial issue(s) without the need for an extension based on all considered under utilised space?
Future proofing
  • what are the future needs for the family or users of the house in terms of either mobility needs (i.e. unable to access stairs or need for a wheelchair in years to come) or family expansion (i.e. additional children or the need accommodate an elderly relative in later years)?
  • is it the right time to carry out house refurbishment or extension works? The design and local approval process can take around 4-5months to complete thus consideration needs to be given as to time of year when the construction works will start i.e. bad weather conditions could well delay the construction works.
Other elements
  • what are the current opportunities for natural light to the newly planned space(s)?
  • what type of design is most preferred for the new space(s) (i.e. contemporary, country-style or minimalist)?
  • what level/specification of finishes are most preferred (finishes can be the most expensive part of the works)?
  • what are the current location of available service connections?
  • is the property in a conservation area or is it a listed building?
  • will the development require a Party Wall agreement (What is a Party Wall Notice & Agreement?)? Do the neighbours know about the intended development?
Planning Application

The design of a house extension may well qualify as 'Permitted Development' (PD) and thus would not necessarily require planning permission, however to be clear ANY project must be discussed with the Local Authority's Planning Department in the first instance by way of a 'pre-planning application'. Should the project be considered as not requiring planning permission it is highly recommended that a 'Lawful Development Certificate' (LDC) is sought in order to formally certify the same for future records which can also be referenced as part of any future conveyancing process if needed. It is not compulsory to have an LDC but there may be times when you need one to confirm that the use, operation or activity named in it is lawful for planning control purposes. More information on LDC's can be found here.

PD rights were temporarily relaxed in 2012 allowing the maximum distance for a single-storey rear extension to a detached house to be increased from 4m to 8m and for a semi or terrace house to be increased from 3m to 6m in length. In May 2019, this temporary relaxation was made permanent, however, should a project wish to take benefit of this increase in rear extension lengths then the project must be subject to a neighbour consultation scheme in which the Local Authority will consult with the adjoining neighbours upon receipt of an application. More information related to Permitted Development Rights and the Neighbour Consultation Scheme can be found via these guidance notes: 

Note that PD Rights are withdrawn in relation to flats and maisonettes and for projects in the following areas:

  • Conservation areas, as mentioned above;
  • National Parks;
  • Areas of Outstanding Natural Beauty;
  • World Heritage Sites; and,
  • The Norfolk or Suffolk Broads.
Building Regulations

In terms of an extension, other than a conservatory, porch, covered yard or carport open on at least two sides, any house extension project will require Local Authority Building Regulations approval prior to the construction works commencing. Refer below for more guidance in relation to the Building Regulations process:

Budgeting for a house extension

When considering an works to an existing house it is imperative to establish a not-to-exceed budget for the project at the earliest stage. This budget must be as comprehensive as possible allowing not only for the construction works but also any professional fees (e.g. architect, building surveyor, cost consultant, environmental specialist, 3D visualiser etc.), insurances (i.e. for the building) and contingency for any unforeseen risks (e.g. reconfiguration of existing underground services, additional demolition etc.). 

It is highly recommended that a cost consultant or quantity surveyor is engaged at the earliest stage to provide an approximate cost estimate for the project based on an architect's sketch design in order to establish whether the identified budget is still suitable for the project in hand.


Whether your existing house requires an extension or more simply a clever and effective re-plan seeking professional advice is paramount particularly in relation to Local Authority Planning guideline and Building Regulations. That professional advice should be via the appointment of an Architect. The benefit of appointing an Architect is that person will be registered with an accredited body and hold the appropriate professional insurance along with the requirement to comply with various codes of conduct. It is recommended that the appointed Architect has experience both in designing similar types of project and in liaising with the Local Authority's Planning Department.