Extension plans for a Rickmansworth house cost £645 drawings + £150 survey (£795 total) with us, fixed. That covers the measured survey, the drawings, the forms and checklists, submission to Three Rivers District Council and the case officer's questions. The council's householder fee is £548 on top. Three Rivers is unusual in publishing actual measurements for extensions, so we can tell you early how deep and how wide your extension is likely to be allowed.
On this page
How deep can a rear extension be in Rickmansworth?
Three Rivers' design criteria sit in Appendix 2 of its Development Management Policies document, adopted in 2013 (DM Policies LDD). For a single-storey rear extension the starting point is a depth of 3.6 metres, or 4 metres on a detached house. The council can reduce that where the extension would harm a neighbour or be unduly prominent.
That is less than permitted development allows under the larger home extension route, where prior approval can permit up to 6 metres on a semi or 8 metres on a detached house. It is also less than many owners expect from London councils. If you want a deeper kitchen extension, two routes are worth comparing:
- A planning application at the 3.6 or 4 metre guide, which is judged on design and gives you room to shape the extension.
- Prior approval for a larger extension, if the house has permitted development rights. Neighbours can object, and the council judges only the effect on them. Our guide to larger home extensions explains how it works.
We check which route your house can use before we draw anything.
What about side and two-storey extensions?
At first floor, Appendix 2 asks for at least 1.2 metres from the flank boundary, to stop houses joining into a terrace. A two-storey extension may sit on the boundary at ground floor if its first floor is set in by 1.2 metres. In low-density areas the gap must be bigger, and in high-density areas an absolute minimum of 1 metre may be accepted. A two-storey rear extension must not cross a 45-degree line drawn across the garden from a point on the shared boundary, level with the neighbour's rear wall.
Appendix 2 also sets privacy distances for new development: about 28 metres between the backs of houses that face each other, and at least 14 metres of rear garden where garden length alone provides privacy. A first-floor rear window close to those distances will need careful placing.
What will Three Rivers ask for with the application?
Three Rivers' validation checklist, adopted on 1 August 2024, has two items that catch out householder applications (validation checklist):
- The Local Biodiversity Checklist, completed with its conclusion. It applies to every application, including householder ones.
- A street scene elevation for any extension that raises the ridge height, showing the buildings either side.
Plans and elevations must be at 1:50 or 1:100, each showing the paper size, the scale and a scale bar. A tree survey is needed where there is a Tree Preservation Order, where the house is in a conservation area, or where trees on or next to the site are over 75mm in diameter. We prepare the drawings and the checklist together. A tree survey is a separate report, and we confirm it at the quote stage.
Is my house in a conservation area?
Three Rivers has 22 conservation areas (council heritage page). Two matter most in and around Rickmansworth:
- Rickmansworth, the historic town. From 1 July to 12 August 2026 the council consulted on a draft Character Appraisal and Management Plan that reviews the area's boundary, its character areas and important views (consultation page). Until the new appraisal is adopted, check the current boundary, and expect the new document to be used in decisions once it is.
- Moor Park, south-east of the town. An Article 4 direction published with its appraisal restricts permitted development rights, so changes that are free elsewhere may need an application there. Any basement in the Moor Park Conservation Area needs a flood risk assessment that covers underground watercourses.
In any conservation area the application needs a heritage statement, from £565 with us, and permitted development rights are narrower.
What if the house is in the Green Belt?
Much of Three Rivers outside its towns is Metropolitan Green Belt, and some houses on the edges of Rickmansworth are in it. Policy DM2 says extensions in the Green Belt that are disproportionate in size to the original building, individually or cumulatively, will not be permitted. It also considers how close the house is to others, whether it is or would become prominent, and whether the extension keeps the Green Belt open. National policy defines the original building as the house on 1 July 1948, or as first built if later (NPPF Chapter 13, GB7).
"Cumulatively" is the word to note. A house extended in the 1980s may have little allowance left. We read the planning history before quoting a design.
If your house is on lower ground near the rivers, check the flood zone too. An extension in Flood Zone 2 or 3 needs a flood risk assessment, from £375 with us.
How long and how much in total?
Three Rivers is quick. In the year to 30 June 2026 it decided 494 of 503 householder applications within eight weeks, prior approvals excluded (MHCLG live table PS2). Before you apply, its householder pre-application advice costs £115, and the council also runs a free duty planner service.
- Our planning drawings and submission: £645 drawings + £150 survey (£795 total).
- Three Rivers' householder fee: £548.
- After approval, building regulations drawings and structural calculations: our package is from £1,245 drawings + £150 survey (£1,395 total) for one extension.
Our Three Rivers page sets out the council's validation rules, and the Hertfordshire page covers the county. For a two-storey scheme in the next village west, see two-storey extension plans in Chorleywood. For a Hertfordshire town with a different council, see extension plans in Borehamwood.
Frequently asked questions
Can I build a 6 metre rear extension in Rickmansworth? Not usually through a planning application, where the guide is 3.6 metres, or 4 on a detached house. Prior approval for a larger home extension may allow up to 6 metres on a semi if the house has permitted development rights.
What is the Local Biodiversity Checklist? A Three Rivers form that every application must include, householder ones too. Without it the council will not validate the application.
Does the new Rickmansworth conservation area appraisal affect me? If your house is inside the boundary, or inside a boundary extension once adopted, yes. We check the boundary in force when we take the job.
How long does Three Rivers take? It decided 494 of 503 householder applications within eight weeks in the year to 30 June 2026.
If the application goes through the Planning Portal, the Portal adds its own service charge: nothing, £99 or £149 including VAT, depending on the application type and complexity, paid to the Portal at submission and not normally refunded with a council fee (Ashford Borough Council, Planning Portal service charges, 11 September 2026). It is not ours and not the council's. Other submission routes exist; where your council accepts one without a charge, we can use it.
