"Hi

The most straightforward option for you right now is to visit or call your local planning department – they should be able to answer a lot of your questions about planning permissions, procedures for planning and the like.

You will need planning permission in some circumstances:
- Covers more than half of the area surrounding your home
- Increases the overall height of the building
- Extends over half the width of the building currently
- Extends above 4m tall
- Extends from the rear of the house by over 6m for an attached house / 8m for a detached house
- Extends the building in the direction of a road
- Involves the use of materials different to the original building
- Features a balcony or raised veranda

If your plans are not beyond these criteria, then you will likely be able to have your extension via Permitted Development Rights – it would be best to check your plans with your local planning authority to check this ahead of any development.

If your extension will require planning permission, then you can download a Provisional Development Enquiry form to fill in. This will require details about the plans, including an approximate drawing/design and the measurements.

You will receive a response to the enquiry in about 7 days from them receiving it. Enquiries themselves are free, however some councils may have a charge of £18 for an administration fee, so check this with your council before sending your enquiry.
Following this, you can consult with an architect to produce your plans. Additionally, consulting with an architect definitely should be ahead of getting quotes from builders. However, it would be advisable that development on-site begins within 8 weeks of the first meeting with the architect.

The costs of the plans will vary by the architect, so you may wish to discuss the cost of their plans before they produce them. There will be a need for good, detailed plans, so that any builder hired can follow the plans precisely less risk of error – in many cases, spending a little extra to make sure your plans will be right (for you and structurally speaking), can make a massive difference to the end result.
For determining the cost of your extension, you could base it, initially, on the idea that the cost would be the floor space, width by length, multiplied by £1300. This should give you an approximate to start from, although eventual costs may vary depending on the builders you may use. Once you have the plans from your architect, you can then seek builders to provide quotes for the extension work.

Typically, the construction process would be (without any significant delays) around 12 weeks to complete an extension properly. For your extension, you should consult with your architect or builder for their estimates of the development, to be aware of any unique factors involved that could alter the timeframes.

Be aware that for any extension, there will be a need to be compliant with Building Regulations (irrespective of it being a permitted development or being given planning permission), which will mean a building control officer will need to visit during the development stages to review the work being done. They will be able to approve the extension as compliant or not. There will be an additional fee for the building control reviews, the cost will vary depending on your local council however."