The Number One Article On Double-storey Extensions
2026.09.26 01:49
Choosing between a single-storey and a double-storey extension is one of the bigger decisions in the planning of any home extension, and it is one that homeowners often make on gut feel rather than analysis. A single-storey extension feels less daunting and less disruptive; a double-storey feels more ambitious and delivers more space. But the actual maths of cost per square metre, planning ease, disruption during the build and future value all deserve honest thought before the decision is locked in.
Single-storey rear extensions are the most common home extension in Bournemouth and across the country, and for good reason. They are almost always the easiest to get planning consent for, either under permitted development or through a Prior Approval application. They do not require major changes to the existing first-floor layout of the house. They deliver the space that most families actually need - a larger kitchen, a family room, a dining space - without complicating the upstairs. The disruption during the build is limited mostly to the ground floor.
Double-storey extensions deliver much more space for a proportionally smaller cost increase. The foundations, the roof and the professional fees are shared across two floors instead of one, and the marginal cost of the second storey is often forty to fifty per cent less per square metre than the first. If the household genuinely needs more bedrooms or a larger primary bedroom suite, a double-storey extension delivers that at a cost per useful room that a single-storey cannot match.
Planning is where the two options diverge sharply. Two-storey extensions have significantly tighter rules under permitted development - maximum three metres deep from the rear wall, minimum seven metres from the rear boundary, matching materials, no balconies, and pitched roof only. Most two-storey projects therefore need a full planning application rather than Prior Approval, and the timeline lengthens by weeks. Neighbours have more opportunity to object, particularly on grounds of overlooking or loss of light. This is not a reason to avoid a two-storey extension - many are approved without objection - but it is a real difference in the pathway.
Disruption during the build is significantly greater with a two-storey extension. A single-storey extension leaves the existing first floor of the house entirely undisturbed - upstairs bedrooms are usable throughout the build, and the household can continue to live in the property with reasonable comfort. A two-storey extension involves opening up the existing first-floor rooms to connect them to the new space, and often means the household needs to move out for portions of the build. Costs for temporary accommodation should be built into the budget rather than discovered mid-project.
The structural implications are also different. A single-storey rear extension needs foundations sized for the loads it carries and connections to the existing house that are competent but not complex. A two-storey rear extension imposes much heavier loads on the ground and often requires structural changes to the existing house - the ground floor wall between the old kitchen and the new extension may need a beam inserted where before it was just a doorway. Structural engineer fees, steel costs and Building Control complexity all step up with a second storey.
Resale value is the argument most often made in favour of two-storey extensions, and it is broadly true but not universal. Adding a bedroom and an en-suite typically returns more than adding a bigger kitchen alone. But the value uplift depends on the local market and on what the property was under-supplied in. A three-bedroom Bournemouth semi in a family area gains more from a fourth bedroom than a two-bedroom flat gains from a bigger living room. Local estate agents will give a candid answer on which type of extension delivers the best return for a particular street.
The honest advice, from any Bournemouth extensions company that is not just optimising for the size of their invoice, is that neither type is universally better. Single-storey suits households whose primary need is more downstairs living space and who want the quickest, cheapest route. Double-storey suits households who need bedrooms and are prepared for a longer, more disruptive project in exchange for significantly more space at a better rate per square metre.
Single-storey rear extensions are the most common home extension in Bournemouth and across the country, and for good reason. They are almost always the easiest to get planning consent for, either under permitted development or through a Prior Approval application. They do not require major changes to the existing first-floor layout of the house. They deliver the space that most families actually need - a larger kitchen, a family room, a dining space - without complicating the upstairs. The disruption during the build is limited mostly to the ground floor.
Double-storey extensions deliver much more space for a proportionally smaller cost increase. The foundations, the roof and the professional fees are shared across two floors instead of one, and the marginal cost of the second storey is often forty to fifty per cent less per square metre than the first. If the household genuinely needs more bedrooms or a larger primary bedroom suite, a double-storey extension delivers that at a cost per useful room that a single-storey cannot match.
Planning is where the two options diverge sharply. Two-storey extensions have significantly tighter rules under permitted development - maximum three metres deep from the rear wall, minimum seven metres from the rear boundary, matching materials, no balconies, and pitched roof only. Most two-storey projects therefore need a full planning application rather than Prior Approval, and the timeline lengthens by weeks. Neighbours have more opportunity to object, particularly on grounds of overlooking or loss of light. This is not a reason to avoid a two-storey extension - many are approved without objection - but it is a real difference in the pathway.
Disruption during the build is significantly greater with a two-storey extension. A single-storey extension leaves the existing first floor of the house entirely undisturbed - upstairs bedrooms are usable throughout the build, and the household can continue to live in the property with reasonable comfort. A two-storey extension involves opening up the existing first-floor rooms to connect them to the new space, and often means the household needs to move out for portions of the build. Costs for temporary accommodation should be built into the budget rather than discovered mid-project.
The structural implications are also different. A single-storey rear extension needs foundations sized for the loads it carries and connections to the existing house that are competent but not complex. A two-storey rear extension imposes much heavier loads on the ground and often requires structural changes to the existing house - the ground floor wall between the old kitchen and the new extension may need a beam inserted where before it was just a doorway. Structural engineer fees, steel costs and Building Control complexity all step up with a second storey.
Resale value is the argument most often made in favour of two-storey extensions, and it is broadly true but not universal. Adding a bedroom and an en-suite typically returns more than adding a bigger kitchen alone. But the value uplift depends on the local market and on what the property was under-supplied in. A three-bedroom Bournemouth semi in a family area gains more from a fourth bedroom than a two-bedroom flat gains from a bigger living room. Local estate agents will give a candid answer on which type of extension delivers the best return for a particular street.
The honest advice, from any Bournemouth extensions company that is not just optimising for the size of their invoice, is that neither type is universally better. Single-storey suits households whose primary need is more downstairs living space and who want the quickest, cheapest route. Double-storey suits households who need bedrooms and are prepared for a longer, more disruptive project in exchange for significantly more space at a better rate per square metre.
