What you could build at DA15 9NN: the planning rules at this address, what neighbours have built and how big, planning decisions nearby, rough costs and the value an extension adds, and how to get started. In Blackfen, Bexley.
56extensions seen within 600 m in the last 10 years
0loft conversions within 600 m in the last 10 years
84.1extensions per 1,000 homes in the neighbourhood (10 years)
● work seen on energy certificates · ● application approved · ● refused · ● pending or other · ● this postcode
The rules at this address
Houses usually have permitted development rights: some rear extensions (typically up to 3 m deep for a semi or terrace, 4 m detached), loft conversions within volume limits, and porches can go ahead without planning permission. Larger single-storey rear extensions (up to 6 m / 8 m) can use the “prior approval” neighbour scheme.
Not in a conservation area (from council registers).
No Article 4 direction recorded for this postcode.
Listed buildings need listed building consent for most changes, inside and out: check the National Heritage List.
To be certain work is permitted development, apply for a lawful development certificate (about £129 for householders in England). Building regulations apply either way.
Across England and Wales, homes extended by a typical 30 m² and then resold rose about 11.1% more than similar homes nearby over the same years (124,884 resales). How extensions pay off by size, type and region.
Single-storey rear or side extension: about £2,000–£3,000 per m² (London £2,500–£4,000), plus fees and VAT
Loft conversion: about £45,000–£75,000 for a dormer; more for a mansard or hip-to-gable
Two-storey extension: about £1,800–£2,800 per m² of new floor
Garage conversion: about £15,000–£30,000
Basement: about £3,500–£5,000 per m², London mostly
Build costs are rough 2026 ranges from industry guides, not quotes. Get three quotes for your plans.
Move or improve?
The money you don't get back either way, for the same extra space in Blackfen.
Move to a home 20 m² bigger (about £531,000)
Stamp duty on the new home
£16,550
Estate agent on yours 1.2% + VAT
£6,048
Solicitors, buying and selling
£3,000
Survey
£600
Removals
£1,500
Money gone
£27,698
Extend by 20 m²
Building, plus 15% for design, planning and contingency
£57,500
Less what it adds to your home's value 60% of the local price
−£66,600
Money gone
−£9,100
On these figures, extending loses about £36,798 less. Moving also changes the area, the schools and the commute; extending means months of building work and depends on planning.
Building costs vary a lot (a single-storey rear extension is often £2,000–£3,000 per m², more in London). How much space adds depends on the home and street: what extensions added to resale prices. A guide, not advice; nothing you type leaves this page.
Getting started
Look at what neighbours did. Open applications above to see drawings that were approved on similar houses, and who designed them.
Check the route: permitted development (with a lawful development certificate), prior approval for larger rear extensions, or full householder planning permission.
Get a measured survey and a designer: an architect, an architectural technologist or a designer who handles planning drawings.
Apply (householder permission is about £258 in England; councils aim to decide within 8 weeks).
Building regulations through the council's building control or an approved inspector, and party wall notices to neighbours if you build on or near the boundary.
Choose a builder: written quotes, a contract, staged payments, and check their insurance and references.
From energy certificates: an extension where a later certificate shows more floor space, a loft conversion where roof rooms appear. Certificates come with sales, lettings and some building work, so the year is when it was first seen.
Not every extension shows here. We only see building work when a new energy certificate is lodged, which happens when a home is sold, let or newly built. A loft or extension added since then won't appear, even with planning permission and building control sign-off, because those records aren't published as open data outside London. Missing work doesn't mean missing paperwork.
Rear extension
Approved
Nov 2025
25/02064/LDCP
338, Blackfen Road, Da15 9ny
Loft or dormer
Approved
May 2025
25/00616/LDCP
31, Wellan Close, Da15 9pn
Rear extension
Approved
Apr 2025
25/00418/GPDE
33, Sherwood Park Avenue, Da15 9hn
Rear extension
Not needed / lawful
Nov 2024
24/02603/GPDE
31, Wellan Close, Da15 9pn
Rear extension
Refused
Oct 2024
24/02335/GPDE
43, Wellan Close, Da15 9pn
Loft or dormer
Approved
Oct 2024
24/02296/LDCP
50, Westerham Drive, Da15 9nr
Rear extension
Not needed / lawful
Jun 2024
24/01569/GPDE
56, Harcourt Avenue, Da15 9ln
Rear extension
Not needed / lawful
Mar 2024
24/00493/GPDE
50, Westerham Drive, Da15 9nr
Rear extension
Refused
Mar 2024
24/00433/GPDE
56, Harcourt Avenue, Da15 9ln
Rear extension
Refused
Feb 2024
23/03428/GPDE
250, Sherwood Park Avenue, Da15 9jn
Other householder work
Not needed / lawful
Dec 2023
23/02866/PRIOR
191, Sherwood Park Avenue, Da15 9jg
Rear extension
Not needed / lawful
Sept 2023
23/01868/GPDE
162, Ramillies Road, Da15 9jf
Rear extension
Refused
Sept 2023
23/01873/LDCP
15, Ramillies Road, Da15 9ja
Rear extension
Refused
Sept 2023
23/01572/LDCP
14, Wellan Close, Da15 9pn
Loft or dormer
Approved
Jul 2023
23/01080/LDCP
89, Ramillies Road, Da15 9jb
Rear extension
Approved
Jul 2023
23/01062/LDCP
103, Tyrrell Avenue, Da16 2bt
Rear extension
Refused
Jun 2023
23/00920/GPDE
45, Penshurst Avenue, Da15 9ez
Loft or dormer
Approved
Jun 2023
23/00054/LDCP
28, Wellan Close, Da15 9pn
Loft or dormer
Approved
Jun 2023
23/00807/LDCP
50, Westerham Drive, Da15 9nr
Rear extension
Approved
May 2023
23/00698/LDCP
103, Tyrrell Avenue, Da16 2bt
Loft or dormer
Approved
May 2023
23/00617/LDCP
74, Sherwood Park Avenue, Da15 9jj
Loft or dormer
Approved
Apr 2023
23/00416/LDCP
3, Penhill Road, Da5 3ep
Loft or dormer
Approved
Apr 2023
23/00250/LDCP
45, Penshurst Avenue, Da15 9ez
Rear extension
Not needed / lawful
Mar 2023
23/00058/GPDE
103, Tyrrell Avenue, Da16 2bt
Rear extension
Refused
Feb 2023
22/03117/GPDE
Householder applications validated since 2023 within 600 m. The council's page has the drawings, the decision notice and who the agent or architect was. Hover a row for the description. Source: GLA Planning London Datahub.
81% (67)
88%
8 weeks
Basement
–
–
83%
9 weeks
Porch or front extension
–
81% (16)
81%
8 weeks
Share of decided applications approved since 2023 (number decided in brackets). Withdrawn applications aren't counted; many are withdrawn when refusal looks likely.