Do I require Planning Permission:

This guidance reflects temporary increases to the size limits for single-storey rear extensions that must be completed by 30 May 2019, and the associated neighbour consultation scheme.

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.

Please note: The permitted development allowances described here apply to houses and not to:

Flats and maisonettes (view our guidance on flats and maisonettes)
Converted houses or houses created through the permitted development rights to change use (as detailed in our change of use section)
Other buildings
Areas where there may be a planning condition, Article 4 Direction or other restriction that limits permitted development rights.
Installation, alteration or replacement of a chimney, flue or soil and vent pipe: Read guidance on the permitted development regime under Class G of the regime.
Please be aware that if your development is over 100 square metres, it may be liable for a charge under the Community Infrastructure Levy.
Permitted Development for householders – Technical Guidanc
You are strongly advised to read a technical guidance document produced by the Government to help understand how permitted development rules might apply to your circumstances.
View 'Permitted development for householders – Technical guidance' on Gov.uk

Do I Require a Building Warrant:

Most extensions of properties require approval under the Building Regulations.

There are a number of classes of new buildings or extensions of existing buildings that do not need Building Regulations approval, i.e. are exempt from the Regulations. Read more about exemptions.

The following pages give an indication of some of the elements normally required to satisfy the requirements of the Regulations when building an extension.

The following common work sections give an indication of several other elements normally required to satisfy the requirements of the Regulations when building an extension:

You must also find out whether work you intend to carry out falls within The Party Wall etc. Act 1996.

It is advised to maybe call

Do I need planning permission?

If you want to carry out building work you may need planning permission.

You should always check with your planning authority about whether you need to apply for planning permission for building work.

Find out whether you need to apply for planning permission by contacting your local planning authority.

Is Planning Permission Required...

You may need planning permission if:

you want to build something new
you want to make a major change to your building – like building an extension
you want to change the use of your building
your building is in a conservation area (you may need 'conservation area consent')
your building is a listed building (you may need 'listed building consent')

Find out if your building is in a conservation area by contacting your Planning Authority.

Find out if your building is a listed building by checking Historic Scotland's listed building search or contacting your Planning Authority.

Contact your local planning authority

Planning application not needed

Smaller building projects that don't impact on the surrounding area might not need a planning application. This is called 'permitted development'.

Permitted development rights are granted so that many instances of small alterations and extensions can be carried out without needing to apply for planning permission.

Find out if the building works you want to carry out meet the rules for permitted development.

What happens next

Once your planning application has been submitted, your Planning Authority will:

publish details of your application on the Planning Authority's website
tell your immediate neighbours that an application has been made

While an application is being considered, anyone can visit the Planning Authority's website to:

make comments on it
track the progress

Planning authorities aim to reach a decision within 4 months of receiving a valid application for major developments and 2 months for local developments.

Find more information on how the planning system works on the Scottish Government website.

You can find more guidance on how to comment on or track an applicationin the ePlanning user guide.

Planning decisions

Once you've applied for planning permission, the planning authority will decide whether to grant planning permission.

How long will it take?

In most cases, planning applications are decided within 2 months.

For unusually large or complex applications the time limit is 4 months. For these types of applications, Planning Authorities should offer to enter into a processing agreement with developers which provides certainty and a project managed approach to decision making.

If the decision takes longer, you can appeal.

How will they decide?

Decisions are based on local development plans and will look at:

the number, size, layout, siting and external appearance of buildings
the infrastructure available – like roads and water supply
any landscaping needs
what you want to use the development for
how your development would affect the surrounding area – eg if it would create lots more traffic

All of the above information is taken from https://www.mygov.scot/planning-permission/what-happens-next/