Who this guide is for
- Homeowners considering a ground-floor extension.
- Anyone planning a rear or side single-storey extension.
- People wanting better connection to outdoor space.
- Readers preparing a brief for professionals.
Room purpose and footprint
Decide what the extension is for and how much space it needs, balanced against the garden or yard you would give up.
- Primary use (kitchen-diner, living, utility, etc.).
- Footprint versus remaining outdoor space.
- How it relates to existing ground-floor rooms.
- Whether walls between old and new open up.
Connection to garden and light
Single-storey extensions often live or die on their link to the outdoors and on daylight, since deep extensions can be dark in the middle.
- Doors and openings to the garden or patio.
- Daylight strategies (a professional topic for rooflights, etc.).
- Threshold and level changes to outside.
- Views and privacy.
Roof, drainage and openings as topics
Note the technical themes for professionals: the roof form, how rainwater is handled, and the windows and doors.
- Roof form (flat, pitched) as a professional topic.
- Rainwater and drainage as a planning topic.
- Window and door positions for light and use.
- How the extension roof meets the existing wall.
Materials and professional review
Capture material preferences and bring the technical scope to professionals.
- Material and finish directions.
- How the extension looks against the house.
- Construction access at ground level.
- Professionals to involve and rules to confirm.
How to use this guide responsibly
Build Design Hub provides educational planning content only. This page does not determine whether a project is feasible and gives no construction, engineering, architectural, structural, inspection, legal, code or contractor advice. Its purpose is to help you think through scope, constraints and questions before qualified professionals assess your specific property.
Feasibility depends on property conditions and professional review. Requirements vary by location and project. Costs vary by scope, materials, access, labor, hidden conditions and jurisdiction; timelines vary by scope, approvals, contractor availability and material lead times. Safety-critical work should be reviewed and carried out by suitably qualified professionals.
- This page helps you plan and prepare — it does not confirm what is possible or permitted.
- Confirm local rules, permits and approvals with the relevant authority and qualified professionals.
- Structure, fire safety, egress/access, ventilation and moisture are professional-review topics.
- Costs and timelines vary widely — treat any figure only as something to confirm with professionals.
- HELPERG LLC operates and publishes Build Design Hub and is not a construction, design, engineering, inspection or legal provider.
Single-storey extension checklist
- 1Define the primary use of the extension.
- 2Balance footprint against remaining outdoor space.
- 3Plan the connection to existing ground-floor rooms.
- 4Plan openings and connection to the garden.
- 5Note daylight strategy as a professional topic.
- 6Note roof form and drainage as professional topics.
- 7Plan window and door positions.
- 8Capture material and appearance preferences.
- 9Note construction access.
- 10Confirm local rules with professionals.
Common mistakes to avoid
- Extending so deep the middle of the room is dark.
- Giving up more garden than expected.
- Treating roof and drainage as DIY decisions.
- Ignoring level changes to the garden.
- Assuming it is permitted without confirmation.
- Fixing cost or timeline before professional review.
When to involve a professional
- Additions and conversions commonly involve structure, the building envelope, fire safety, egress/access, ventilation and moisture — all of which need qualified design and professional review before work.
- Whether a project is permitted, and what approvals it needs, varies by location — confirm with the local authority and qualified professionals; this page makes no legal or code claims.
- Roof form, drainage and how the extension meets the existing structure need qualified design and review.
- Build Design Hub does not determine feasibility or provide construction, engineering, architectural, inspection or contractor advice — use this page to prepare, then have qualified professionals assess your property.
- Requirements, permits, costs and timelines vary by location and project; confirm specifics with qualified professionals and the relevant local authority.
- Safety-critical work — structural, electrical, plumbing, gas, roofing, waterproofing, ventilation, insulation and fire safety — should be designed and carried out by suitably qualified professionals.
Frequently asked questions
Questions readers ask about this topic
How big can a single-storey extension be?
That depends on the property and local rules, which vary — this page makes no claims. Plan the footprint you want and confirm what is possible with qualified professionals and the local authority.
How do I keep a deep extension bright?
Daylight in deep single-storey extensions is a design challenge often handled with rooflights and openings. This guide does not give technical solutions — raise it with an architect or designer.
Do I need permission for a rear extension?
It varies by location and property, and this page makes no legal claims. Confirm with your local authority and qualified professionals.
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