House extension architects in Brighton and Hove

What will Brighton and Hove planners look at in a rear extension?

A house extension here is judged mainly on two questions: does it sit comfortably behind the original building, and does it leave the neighbours' habitable rooms with light and outlook. The council's Supplementary Planning Document 12 says rear extensions should be subservient to the host property and should keep the bulk along a shared boundary to a minimum (SPD 12). City Plan Part Two policy DM21 sits behind it and grants permission for extensions that are well designed and scaled, respond to the area's character and use materials that complement the parent building (City Plan Part Two).

The guide does not give a single maximum depth. It sets design tests instead, and the ones it lists for a rear extension are specific:

• The extension must comply with the 45 degree rule, which the guide describes in its Appendix 3.
• It must not overhang a neighbour or replace the boundary wall or fence, unless the neighbour already has an extension and losing the wall helps maintenance.
• In the majority of cases, single and two storey extensions should not extend beyond the main side walls of the building, including wings and projections.
• The ridge of a pitched roof over a single-storey extension should sit lower than the cill of the first-floor windows, though the guide accepts flat roofs where they integrate well.
• Side-facing windows should generally be high level, fixed shut or obscurely glazed.

How does the 45 degree test work?

The guide takes the test from the Building Research Establishment's 2011 daylight and sunlight guidance. You mark the centre line of the neighbour's affected window on a plan and draw a 45 degree line from the outermost part of the extension towards that window. If the line passes beyond the centre point, the guide says there is a chance daylight will suffer. It applies to habitable rooms rather than bathrooms, halls or landings, and it works best for extensions at right angles to the window. A scheme that fails may still need a fuller daylight study, so ask an architect to run the line on the survey drawing before design starts.

Where do Brighton's terraces make extensions harder?

In the Council Tax valuation lists, 39.6% of Brighton and Hove homes were built before 1919 (VOA, 31 March 2026). The guide's own illustrations include a high density Victorian terrace in Hanover, detached homes in Hangleton and semis in West Hove, and it says some areas, particularly elevated ones, are more sensitive to change than others. The rear of a hillside terrace is often visible from the street behind, which matters because the guide calls for particular care with end-of-terrace and corner houses.

Side and infill additions are the other typical Brighton move. SPD 12 asks for side extensions to be set back from the front elevation and, for two storeys, set down from the ridge, to avoid a terracing effect. For infill extensions it asks that the bulk along the shared boundary stay as close to 2 metres in height as reasonably possible. Windows, eaves, gutters and downpipes should be kept off party walls so the neighbour's future extension is not blocked. Party walls are a separate civil matter, covered by the Party Wall etc. Act guidance on GOV.UK.

What changes inside a conservation area or an Article 4 street?

The council's conservation areas page lists 34 areas and warns that you may need permission for minor changes (conservation areas). Section 5 of the extensions guide is blunt about historic buildings:

• Side extensions and rear infill extensions will not be acceptable where they would lose the symmetry of a historic building or group, or disrupt the original plan form.
• Front extensions are unacceptable in principle, and porches are not acceptable unless the building was originally meant to have one.
• Rear flat roofs behind pitched-roof buildings may be acceptable where that is typical of the terrace.
• Rear elevations that are not publicly visible get a more flexible approach, particularly where past alterations have already eroded the group's significance.
• The council expects material samples and design details where appropriate, to be approved with the application.

Article 4 directions go further. The council's summary of The Avenues direction, approved in March 1991, says the enlargement or alteration of a property, porches, sheds and hardstandings all need planning consent on every property it covers, and that small extensions and conservatories are listed among the additions needing consent (The Avenues direction). Ask the planning team whether the direction on your street has since been amended.

What does the council ask you to submit?

The householder checklist for a single-storey or two-storey extension lists the application form, a CIL additional information form, a site location plan, a block plan, existing and proposed floor plans and elevations, a biodiversity checklist, and a tree plan if building work is within 5 metres of a tree. A design and access statement is needed only if the site is in a conservation area and the new floor space is 100 square metres or more (householder checklist). A heritage statement is listed for archaeological sites or areas, and the council's heritage page separately expects one where a conservation area or listed building is affected.

Two Brighton details catch people out. First, the biodiversity checklist is the council's wildlife assessment check, which you complete online and attach, and the council says it cannot validate a householder application without it (wildlife check). Second, extensions to a small house in multiple occupation, and extensions to flats, use the full application route, not the householder form (checklist for full applications).

None of this is the planning fee, which is set nationally. The council adds that extra fees apply if the building is over 11 metres tall or the work is not built to common industry standards. If you would rather use a registered building control approver, ask them for their own quote.

Is it worth paying for pre-application advice on an extension?

It depends on how close the scheme is to SPD 12's tests. A written response costs £154.50, and a site meeting with a written response £237.16. Where the house is listed or in a conservation area and the council agrees heritage input is needed, the same options cost £408.54 and £860.15 (household pre-application advice). For a modest rear addition that clearly passes the 45 degree line, you may reasonably go straight to an application. For a side infill in a conservation area, an informal view on symmetry and materials can save a refusal.

Our wider material covers the basics: house extensions nationally, the planning rules for extensions, and the Brighton hub with the city's figures. For the application route itself see Brighton planning applications.

What should you ask an architect before they draw your extension?

1. Confirm they are on the Architects Registration Board register at arb.org.uk.
2. Ask them to run the 45 degree test on your neighbours' windows before sketching, and to show it on the drawings.
3. Ask which conservation area, Article 4 direction or listing applies, and whether they have read its character statement.
4. Ask who prepares the wildlife check and any heritage statement, and whether it is in the fee.
5. Get written quotes on one brief so you can compare like with like.