Worked example: Illustrative 30m² wraparound · UK average · standard access
Quick budgeting answer: Our UK-average model places this example at £119,000, with lower and higher construction-rate scenarios of £89,500–£149,000. These are planning allowances, not a narrow predicted contractor quote.
Illustrative model results for the precise example shown, not current quotes from builders. Budgets include an assumed 20% VAT on construction, professional/approval allowances and 10% contingency. A survey or different specifications could change them.
Why this type of extension can be complex
The example is represented by a 6m × 5m floor area in our model, with a 13% illustrative wraparound complexity factor applied to the basic building rate. That factor is our modelling assumption, not a published industry tariff. Your actual price depends on the geometry, amount of side return, neighbour conditions and the extent of wall removal.
Four design decisions worth pricing early
Linking rear and side sections may require different falls, rooflights and careful waterproofing at awkward junctions.
A wide kitchen-diner opening can require structural beams and support during works.
Side returns often contain drains, gullies or service routes. Ask for a survey and a buildable drainage strategy.
Narrow paths, shared walls and party-wall matters can change access, programme and professional costs.
What to specify for a fair comparison
- Show builders the same measured existing-plan drawings and proposed L-shaped layout.
- Include the complete roof geometry and rooflight specification, not just a flat area figure.
- Clarify any removal of load-bearing walls, steel design and temporary propping.
- Identify drains, utility routes and whether side access will be available during construction.
- List the finished-room standard: glazing, heating, floor finishes, kitchen work and decoration.
Consider the planning position before committing
Wraparound designs can exceed straightforward permitted-development criteria, particularly near boundaries or with more than one direction of extension. In England, seek confirmation from the local planning authority or a qualified planning professional. Also check building regulations and whether the Party Wall etc. Act affects adjoining owners.
Ask your designer to compare a simple rear extension with the wraparound alternative. You may discover that a slightly smaller addition achieves the same kitchen layout with fewer structural complications.
Calculate your own extension budget
The examples above describe specific assumptions. Change the dimensions, region, finish and extras to explore your project.
Calculate my extension price →Common questions
Is a wraparound extension more expensive than a normal rear extension?
Often, yes for the same nominal floor area, because the roof, junctions, drains and structural openings can be more complicated. Our model applies an illustrative wraparound uplift, not a fixed contractor surcharge.
Does a wraparound extension need planning permission?
It frequently needs careful assessment because side and rear extensions interact with different permitted-development conditions. Do not assume combining them keeps the whole proposal within permitted development.
What if a drain runs through the side return?
The designer may need a drainage survey and an agreed solution. Allowing for a diversion or build-over approval can change timing and budget.
Pricing context: Federation of Master Builders construction benchmark; the full-budget numbers are generated by our published illustrative model, which makes additional assumptions. For permissions in England see Planning Portal. Other UK nations have different rules. Always obtain site-specific professional advice and comparable written builder quotes.