A practical guide to loft conversion planning permission across Barnet borough and the Hertsmere area — when you can use permitted development, when you need an application, and how the process actually runs.
Two Routes: Permitted Development vs. Planning Application
Most loft conversions in Barnet fall under permitted development (PD) — a national set of rules that lets you extend without applying for planning permission, provided the work stays within strict size, height and position limits. The rest need a full householder planning application to the council.
Permitted Development Limits — Lofts
Your loft conversion is likely permitted development if all of the following are true:
- Volume added: ≤ 40m³ (terraced house) or ≤ 50m³ (semi or detached)
- No part of the extension is higher than the existing roof's highest point
- No raised platforms, verandas, balconies or roof terraces
- Rear dormers don't extend beyond the existing rear wall plane
- Side-facing windows are obscure-glazed and non-opening below 1.7m
- Materials are similar in appearance to the existing house
- The house isn't in a conservation area, designated AONB, or subject to an Article 4 direction
When You Definitely Need Planning
- Mansard conversions — change the roof profile, exceed volume thresholds
- L-shaped dormers — usually exceed PD volume
- Conservation area properties — Article 4 directions and conservation designations remove PD rights for lofts. Affected areas include parts of High Barnet (EN5), Whetstone Conservation Area (N20), parts of Mill Hill (NW7), and Borehamwood (parts of WD6).
- Listed buildings — any external change needs Listed Building Consent regardless of PD
- Flats and maisonettes — PD rights for loft conversions are restricted
Which Council Decides?
- London Borough of Barnet — postcodes EN4, EN5, N2, N3, N10, N11, N12, N14, N20, NW2, NW4, NW7, NW9, NW11
- Hertsmere Borough Council — postcodes WD6 (Borehamwood, Elstree), parts of EN6 (Potters Bar)
- Welwyn Hatfield Borough Council — other parts of EN6
The Planning Process
1. Pre-application advice (optional but recommended). Barnet planning offers a paid pre-app service — a planning officer reviews your proposed scheme and gives a written view on whether it would likely be approved. Costs £200–£400 and takes 4–6 weeks. Worth it for any non-trivial scheme, particularly conservation areas.
2. Design and drawings. Your architect or loft conversion specialist produces full planning drawings — site plan, existing/proposed elevations, sections, and (often) a Design & Access Statement.
3. Submission. Plans are submitted to the planning portal. Council validates within 5 working days, then a statutory 21-day public consultation period runs. Neighbours within 50m are notified.
4. Determination. Decision target: 8 weeks from validation. Most household applications hit that target. Conservation areas and complex schemes can extend to 13 weeks or longer.
If Approval Is Refused
Two options: (1) amend the scheme and resubmit (free if within 12 months and the changes meet the planning officer's reasons for refusal); (2) appeal to the Planning Inspectorate (free, but takes 4–8 months and the original decision is upheld in roughly 60% of household-scale cases).
Building Regulations — Separate from Planning
Regardless of whether you need planning permission, every loft conversion needs Building Control sign-off. This covers structural calculations, fire safety (protected staircase, escape routes), insulation to Part L, electrical Part P, and final inspection. Allow £700–£1,400 for Building Control fees and inspections.
How We Help
The loft conversion the specialist we work with handle the full process: planning submission, party-wall agreements, structural calcs, Building Control. You get one point of contact and one combined quote. Tell us your postcode and house type and we'll connect you with someone who's been through Barnet (or Hertsmere) planning many times.
Frequently Asked Questions
How do I know if my house is in a conservation area?
Check the Barnet or Hertsmere council planning map online — they publish conservation area boundaries. The loft specialist will also check on the initial survey.
Can I start work before planning is approved?
No — starting before approval is a planning enforcement offence. Wait for the formal decision notice before commissioning works. You can do design and structural work in parallel with the planning application.
What's the cost of a planning application?
Barnet's householder planning application fee is around £258. Add £1,500–£3,500 for the architect/specialist's drawings and submission work. Pre-application advice is another £200–£400 if you commission it.
Can I object if my neighbour applies for a loft conversion?
Yes — during the 21-day consultation period anyone can submit comments to the council. Comments are weighed against the council's planning policies, not just personal preference. Material considerations include loss of light, overlooking and design impact.