Worked example: 20m² single-storey rear addition · UK average · bungalow-style example
Quick budgeting answer: Our UK-average model places this example at £70,500, with lower and higher construction-rate scenarios of £52,500–£88,000. 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.
What the illustrated price does — and does not — cover
This is our ordinary 20m² single-storey rear-extension model, used as an initial bungalow-style reference. The calculator does not currently include a dedicated bungalow building type, so it cannot separately model a complex hip-to-gable roof rebuild, major internal structural work or adapting an older bungalow’s foundations. Those need drawings and individual pricing.
Roof connections can change everything
Matching the existing roof structure and tiles may be more involved than constructing a simple new flat roof.
Insulation, water runoff, roof falls and the junction with existing coverings need careful specification.
New drainage, ventilation, heating circuits and privacy requirements can add to a shell cost.
Keeping one consistent floor level may require particular foundation, threshold and drainage details.
Questions for a designer or builder
- Is the current roof structure sound, and how will the new roof connect to it?
- Can the proposed floor remain level with the existing bungalow without drainage compromises?
- Will the new layout need a wide structural opening, or can the existing door be reused?
- Which heating, plumbing and ventilation changes are included in the specification?
Compare the alternatives on drawings
A rectangular rear addition might be the simplest way to gain a dining area. A side addition may improve the bedroom arrangement, but access and boundary positions will influence feasibility. If you are also considering converting the roof space, treat that as a distinct project rather than assuming an extension budget covers loft work.
Use the calculator for a basic area-led starting point, then ask a professional to investigate roof design and the condition of the existing bungalow before relying on the figure.
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
Does a bungalow extension cost less than extending a house?
Not automatically. There is no upper-floor structure to connect, but roof alterations, foundations, drainage and a complex junction with the existing bungalow can make a project expensive.
Is this calculator specifically calibrated for bungalows?
No. The displayed example uses the general single-storey rear-extension model as a planning illustration. It does not automatically price bungalow-specific roof alterations or the condition of an older structure.
Can a bungalow extension be built without planning permission?
Some additions to eligible houses in England may qualify as permitted development, but specific restrictions still apply. Listed buildings, designated areas and prior alterations can change the answer.
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.