Householder design guide SPDs: what officers check line by line

A householder design guide SPD is the document your case officer has open in the next tab while reading your extension drawings. It turns the one-line design policy in the local plan into measurable checks: how far a rear extension may project before it crosses the 45-degree line, whether a two-storey side extension must be set back from the front wall, how far a new first-floor window must sit from the neighbour's. If your drawings satisfy those checks, the officer's report is short and favourable. If they do not, the report writes itself the other way, and the refusal will quote the SPD paragraph numbers back at you.

What a householder design guide SPD is and how much weight it carries

Most councils in England adopted their householder design guides as supplementary planning documents under the Town and Country Planning (Local Planning) (England) Regulations 2012. An SPD is not part of the development plan, so section 38(6) of the Planning and Compulsory Purchase Act 2004 does not make it the starting point for a decision. It is a material consideration, and in practice a heavy one, because it explains how the council interprets its own design policy.

The National Planning Policy Framework (August 2026) no longer talks about SPDs at all. Its plan-making chapter refers to supplementary plans (policy PM4) and its design chapter to local design guides, design codes and masterplans (policy DP2). An SPD adopted years ago does not vanish because the vocabulary has moved on. It remains a material consideration until the council withdraws it or replaces it, and officers keep applying it. Check the adoption date on the cover and the local plan policy it says it supports. If that policy has since been replaced, the guide's weight drops, and you can say so in your application.

You will usually find the guide on the council's planning policy page, listed under supplementary planning documents or design guidance. Download the whole thing rather than reading the summary. The checks that decide applications are in the annexes and diagrams, and they are the same checks the officer will run when the case lands on their desk.

Subordination, set-backs and ridge heights

Every householder guide repeats a version of the same principle: an extension should read as an addition to the house, and the house should still look like the house. Officers call this subordination. The guide then expresses it as dimensions, and those dimensions are what you need to design to.

The set-back and the drop in ridge height are the two rules officers apply most rigidly, because they prevent a pair of semis merging into one wide frontage. The first-floor gap to the boundary is the third, and it exists to stop the "terracing effect" on streets where the spacing between houses is part of the character. The precise figures differ between councils, and a guide will often describe them as a starting point rather than a limit. Read the wording. "Normally" and "should" leave room for a justified departure; "will not be permitted" does not.

Roof form is where many otherwise careful applications go wrong. A flat-roofed two-storey side extension on a street of hipped 1930s roofs will be called out by the guide, and no amount of matching brick will rescue it.

The 45-degree and 25-degree amenity tests

The 45-degree test is the workhorse of householder guides. Take the nearest habitable room window on the neighbour's rear elevation. From its centre, or from its nearest edge (guides differ, and you must use the version in yours), draw a line at 45 degrees across the plan. If your extension crosses that line, the guide treats it as likely to cause an unacceptable loss of light or outlook to that room. Some guides add a second 45-degree line in section, drawn from the centre of the window on the elevation, so that a tall extension close to the window also fails.

The 25-degree test comes from BRE 209 (2022), Site Layout Planning for Daylight and Sunlight. It is a screening tool: from the centre of the lowest affected window, a new obstruction that rises above a line drawn at 25 degrees from the horizontal may reduce daylight to a degree that needs a fuller check. Householder guides borrow it mainly for two-storey rear extensions and for flats with windows facing the site.

Neither test is law. An officer can and does weigh other things: which way the window faces, whether the room is a bathroom rather than a bedroom, whether an existing garage already blocks the view. The point is that you should draw the lines yourself before the officer does. Put the 45-degree line on your block plan with the neighbour's window position marked, and state in your application whether the extension clears it.

Materials, fenestration and separation distances

The materials section of a guide is short and easy to satisfy: match the brick, the bond, the mortar colour, the roof tile and the window proportions of the host dwelling. Where you cannot match, say why. A modern glazed rear extension can be acceptable precisely because it is honest about being new, but the guide will still expect the visible front and side elevations to defer to the original.

Windows get more scrutiny than materials. Guides commonly require first-floor side windows to be obscure glazed and non-opening below 1.7 m above the floor, which mirrors the condition in the General Permitted Development Order for permitted extensions. Then come the separation distances. Figures of around 21 m between facing habitable room windows, and around 12 m from a habitable window to a blank flank wall, have appeared in council guides for decades. They are back-to-back privacy standards for houses on conventional plots, and the guide will tell you whether they apply to extensions or only to new dwellings. If your rear extension brings a new first-floor window closer than the standard, expect a request for obscure glazing or a redesign.

Front extensions, porches and boundary treatments

Guides are consistently strict about the front. A porch outside an external door may be permitted development under Schedule 2 Part 1 Class D of the Town and Country Planning (General Permitted Development) (England) Order 2015 if its ground area is no more than 3 m², no part is more than 3 m above ground level and no part is within 2 m of a boundary with a highway. Anything bigger is a planning application, and the guide will assess it against the building line. Where a street has a continuous frontage, an extension projecting forward of it is one of the most common householder refusal reasons.

Front boundary walls and fences over 1 m next to a highway also need permission, and the guide will normally ask for materials that match the street. Hard standing in the front garden is usually permitted development if it is permeable or drains within the curtilage, but the guide may also apply the council's parking standards to what is left of the garden. If a car space is at stake, the parking standards SPD explained in our guide to evidencing parking provision matters as much as the design guide.

And do not assume the permitted development route is open. In many conservation areas an Article 4 direction removes it for front elevations, a point covered in householder Article 4 directions in conservation areas.

Turning the SPD into a short design justification

The guide is a checklist, so treat it as one and hand the officer the completed version.

1. Download the current guide and note the adoption date and the local plan policy it supports.
2. List every numbered guideline that touches your scheme: subordination, set-back, ridge height, rear depth, 45-degree test, windows, materials, boundary treatment.
3. Against each one, write a single line: complies, with the dimension, or departs, with the reason.
4. Annotate the drawings with the same dimensions so the officer can check without a scale rule.
5. Where you depart, give the evidence: the neighbour's window serves a bathroom, the ground slopes away, the house next door already has a matching extension with permission.

That table, with a paragraph of context, is the core of a Planning Statement for a householder scheme. It is also the document the council's validation officer looks for when the local list asks for a design justification, which is why it pays to read the council's validation checklist alongside the design guide before you submit. If a departure is large, a pre-application enquiry can settle it in writing, and the council fee bands show what that costs.

Illustrative example: a rear extension on a 1930s semi projects 4 m at single storey. The neighbour's nearest window is a kitchen 2 m from the shared boundary. A 45-degree line from the centre of that window crosses the extension at about 3.5 m. The applicant either pulls the depth back to 3.5 m, or shows on section that a flat roof at 3 m keeps the extension below the neighbour's 25-degree line, and explains that the kitchen also has a side window. The officer now has a reasoned case rather than a bare breach.

PlanScope can draft that Planning Statement from your postcode, your site photo and your answers about the extension, structured to the policies the council will cite and flagged where a fact still needs confirming. Enter your postcode on PlanScope to see which reports your extension is likely to need. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.