Feasibility/Initial Design

Initial Considerations

The process of designing your extension will naturally evolve from early sketch ideas to a final agreed layout suitable for a planning application and eventually to comprehensive working drawings.

The first stage of any extension project is to find out if your home is suitable to extend and to obtain some initial design ideas on what may be possible. We have termed this the feasibility stage although you may come across numerous other terms for this initial process such as a property assessment or viability study.

Even for straightforward extensions, it is important to be sure that you have looked at all of the options open to you and gathered as much information as possible before you proceed with a specific design. It will cost time and money to change your design later on and, when all the work is completed, you should be satisfied that you have the best solution for your needs.

We would strongly recommend that you use an experienced architectural designer to carry out your initial scheme design as they will be familiar with the limitations that are imposed by planning and building regulations. They will also know how to make the most of the space available.

myhome mygarden can provide you with a comprehensive Feasibility Study for any proposal that you are considering.

At the feasibility stage, you should be considering the overall design of your extension and not be overly concerned with details such as your choice of kitchen units or finishes.

The main considerations you will need to think about at this stage are listed below.

A. Your Requirements

Often know as the 'Client Brief', your first task is to define what you actually require from the extension. It is a good idea to involve the whole family in this process as people may have different priorities. It may also help to speak to someone you know who has had work completed recently as they will have gone through this process.

You will need to decide on the number and use of the rooms you require. Think about rooms that are not used at the moment and whether they can be changed to suit your current requirements. Is the space currently in your property being used to the best effect?

Do you want to make existing rooms bigger? For example, an existing small bedroom could be enlarged by extending it into the new addition. You may be looking to add a room that is multifunctional, such as an occasional guest room, which you can use as a study or TV/Play room for the rest of the time.

Most designers like to have a clear Client Brief from you so that they can help you to create the space you need. Create a list of priorities, dividing it into essential works and desired, but not essential works.

B. Position, Shape and Size

One of many decisions you will have to make is where you would like to build your extension. It is important to ensure that the property remains balanced upstairs and down, especially if you intend to sell in the future. For example, it’s hard to sell a property that only has two bedrooms, but three reception rooms downstairs or four bedrooms upstairs, but only one reception room.

The overall size and position of your extension will be limited by boundaries, planning constraints and cost but does the extra space need to be a certain size for a specific reason?

If you are creating a dining room, for example, have you worked out how big the table will be and how many place settings you require? If it’s an office, you will need enough room for a desk, chair and storage. If you are looking at creating a new bedroom, does this need to be double or single and will it have an en suite?

Our best advice is to plot furniture on to your initial plans - if your designer is using a modern CAD package, this will be normally be easy to do or you could always use the cardboard cutout method!

Also, think about the shape of your proposed extension. Most will be rectangular or square, simply because this is easier and more cost effective to build. If you are thinking about a curved wall or an interesting shape, just be sure you are aware of any cost implications and make sure your furniture will still fit.

It is important to consider your neighbours and what effect your plans will have on them – they may lose sunlight or become overlooked. Try to minimize the impact and talk to them before you seek planning permission. If you have adjoining properties, your neighbours might even be interested in extending in the same way, so you could cut the costs.

At the feasibility stage, your designer should carry out a measured survey of the property prior to preparing scaled plans for you but we would also recommend using a designer that can illustrate your proposal in 3D as this will help you (and your neighbours) to get a good understanding of how the proposal will look.

C. Number of Stories

You will need to consider whether it is best to build single storey or two stories.

Single storey extensions are easier to build and many do not require planning permission (see our factsheet on Planning Permission for Extensions). They also have less impact on the original character of the building.

Double storey extensions can be more difficult to get planning permission for and more disruptive to build but they generally work out cheaper per square metre of floor area than single storey extensions.

Also, consider the use of the rooms and how access will be gained into the extension - It is more usual, for example, for bedrooms to be at first floor level. Access into first floor extensions will often mean losing some existing first floor space unless there is a window off the landing that can be replaced with a door or extended landing area.

Alternatively, think about creating a single-storey extension with the option to add a second storey at a later date (i.e. ensure foundations, joists, lintels, etc are strong enough). It will make it cheaper for you or a buyer of your property to build a second storey at a later date if you have built the ground floor adequately.

D. Light/Ventilation

This an area that your designer should be able to help you with but be aware of the Building Regulations limitations on glazed areas. You will generally be limited to an overall amount of glass so you will need to decide where this will benefit the design most.

There are ways of compensating for excessively glazed areas but they can add to the cost of the works so get professional advice early on.

It is not uncommon to find yourself with planning permission for a highly glazed extension only to discover that you can not then obtain building regulations approval for the design.

E. Structural Alterations

When carrying out your initial design, think about how much of the existing structure you wish to demolish. Whilst most walls can be demolished and steels beams installed, these can be expensive so you may be better off trying to keep as much of the original structure as possible.

A well prepared feasibility study will give you a budget cost for the work that will highlight this area.

F. Landscaping

Will your proposal involve felling any trees? The local council is likely to want to keep the landscape as similar to the existing as it can, so what might seem a small consideration to you – taking down trees or hedges – can be a major factor in your planning application.

If you can, when considering your home extension, try to minimise the number of trees and hedges you remove, particularly any that are near the boundary that faces onto the public highway

G. Budget/Value Considerations

There is sometimes a reluctance to talk about budgets at an early stage, but this can often be the single most important factor that determines your final design.

Whilst you will be building the space for yourselves, we would always recommend that you consider the resale value of your project .The extension will need to blend in well with the existing property and the overall effect of the addition should enhance the whole property. Homeowners like large well-lit spaces and a good balance between ground floor and first floor accommodation.

Try and establish a ceiling price for your property in the local area as there will normally be a limit on how much your property will sell for.

When planning your budget, you will need to try and get the balance right between underspending and overspending. If you are fortunate enough to live in an expensive property, you will probably find the standard of fittings and finishes is high so you will need to match these in order to maintain the property value.

Equally, overspending on a property can make a home look peculiar. For example, you are unlikely to see a return on spending £40,000 on a kitchen in a property that is worth £100,000.

Your feasibility process should provide you with an estimate of the cost for your proposal so you can check this is within your budget. The last thing you want is to end up with a superb design with planning permission only to find that all your builders quotations are twice your allowance. Most designers will find it helpful to know your budget so that they can design your extension within it.

Our section on Extension Costs provides lots of information on cost influences that you will need to consider.

We would always recommend that you allow a contingency (we would suggest 10-20%) for the unknown when working out your costs at the feasibility stage.

H. Planning and Other Approvals Considerations

At this stage, make sure your designer is aware of any local planning policies that may restrict what you can build.

Have other properties on the same street been extended or changed from their original structure and are there any similar to your own?

Your proposal may create a precedent for planning permission and this could cause the planners to look at the application more closely.

Also make sure you have considered any Building Regulations implications, Party Wall requirements and restrictive covenants that may impact on your design (we provide various factsheets on these subjects which can be accessed from our Factsheets index).

Our advice is for you or your designer to have an informal discussion with your local planning office prior to making any submission as they will normally be pleased to give you guidance that could save you time and hassle in the long run.

I. Project Time Considerations

At the feasibility stage, we recommend that you give consideration to the overall time your project will take.

Do you have a set date that the work needs completing by? You may, for example, be planning to work from home at a certain time or you may have an imminent new arrival to the family.

Our section on Project Timing covers this in more detail.