1. Extension Process

Most house extensions follow two stages:

• Planning Application: Hire an architect, carry out a measured survey, prepare existing/proposed drawings, and submit to the council.

• Building Regulations: Prepare technical drawings, obtain structural calculations, get builder quotes, and begin construction.

2. Timeframes & Costs

• Drawings are usually produced within 2–3 weeks of the survey.

• Planning decisions typically take 10–16 weeks due to council backlogs.

• Costs to consider: architect fees, structural engineer fees, council planning fees, and building control fees.

3. Planning Permission & Permitted Development

Whether work falls under permitted development depends on many factors such as:

• Extension position (front/back)

• Garden coverage

• Boundary distance

• Neighbour privacy/light

• Roof volume impact

• Size relative to the original house

Article 4 restrictions

• Loft conversions, single-storey extensions, conservatories, and garage conversions may or may not require permission depending on these rules. Many applications still benefit from submitting for a Lawful Development Certificate.

4. Validity of Permissions

• Planning permission: valid for 3 years from decision date.

• Lawful Development Certificate: valid unless planning policies change or additional alterations affect eligibility.

• Even for permitted development, a council application is required for formal confirmation.

5. Design Changes

• Post-approval changes can be made but require a design amendment application.

6. Building Regulations

• Building Regulation drawings include detailed plans, sections, foundation details, roof details, and steelwork layouts.

• Building Control approval is legally required, even if no planning application was needed.

• Approval can be obtained by Full Plans Submission or via Building Notice, though Full Plans offers more clarity.

7. Project Costs

• Cost estimation before drawings is difficult because materials and design details vary. Exact builder quotes require full Building Regulation drawings.

8. Retrospective Issues

• If work was completed without permission, councils may require a retrospective planning application.

• Missing building control approval can sometimes be remedied by inspections, though compliance is not guaranteed.

• Buyers or sellers of properties with unapproved extensions often need retrospective applications.

9. Planning Timelines

• Standard applications: 8 weeks

• Large or complex projects: 13 weeks

• Validation delays and planning officer backlogs are common.

10. Design & Access Statements

Required for:

• Developments in designated areas (AONB, Conservation Areas, Greenbelt) over 100 sqm

• Listed building consent applications

• Major developments

They explain the rationale behind the design.

11. Listed Buildings

• Listed buildings require Listed Building Consent for almost all internal or external changes.

• Undertaking work without consent is a criminal offence.

• Grades include Grade I, Grade II*, and Grade II.

12. Typical Build Costs (Approximate)

• Costs vary by materials, specification, and builder pricing.

Project Type: Average: Medium: High:

Single Storey Extension £65k £85k £110k

Double Storey Extension £120k £135k £160k

Loft Conversion £65k £85k £110k