A good starting point is the consideration that you want to make sure that whatever you build offers a return on your investment. While it may simply be that you family are out growing the space, it is always sensible to ensure that the space you are adding offers a return. What else should you consider?
It is good to always think practically. If you are adding more bedrooms to your property, then you should also consider additional bathrooms. If you are planning a double-storey extension, then you also need to consider access to the additional space upstairs. Are you increasing the size of your property so much that you should also consider additional parking? These are all elements that will need to be carefully thought out prior to requesting planning.
The cost of your extension is going to increase as the size does. There are guidelines on the average cost per/m2. These may range between £1000 to £2000, depending on the preparatory work required and standard of finish. The higher the standard of specification, the more you will pay. It is also important to remember that if you are considering a double-storey extension it will not cost that much more per square meter. Why is this? The cost of the foundations and roof are required whatever for a single-storey extension. Adding another storey means the addition of walls and floor joists but a proportion of the additional costs can already be attributed to those already agreed for a single-storey project.
Planning permissions may not always be needed for your extension project. However, it is always prudent to seek advice in relation to this. You may feel that your plans fit under the permitted development rights umbrella, but it is important at this stage not to make assumptions. It may delay the process slightly but the peace of mind that you will get from going through the correct channels is well worth it. There may be individual stipulations on your property or neighboring properties that you are not aware of.
Further down the line you also need support with meeting building regulations. These form a separate process to planning permissions and are required for building projects. These rules, put simply, have been formally agreed to ensure that minimum design and construction standards are met. Local authority representatives will visit the property regularly to ensure that the work completed meets these standards.