Will a new building next door block my sun? See the hours you would lose, window by window (2026)

Last checked: 22 October 2026. Written by the Flatwalk team.

Short answer. It depends on how tall the new building is, how far away it stands and which way your windows face. A block to your south takes the most in winter, when the sun is low; a block to your west takes your evenings all summer. Find its height on the planning drawings, then add it to a free sun tool at your address and read the hours each window loses. In the UK, "loss of light or overshadowing" is something the council can weigh, so put those hours in your comment on the application.

Try it free: type your address, add the new building, see the hours you lose

How much sun can one new block take?

A worked example in London. A first-floor window, 4.5 m above the ground, looks across a road at a two-storey building 7 m tall, 20 m away. A developer plans a six-storey block, 19 m tall, in its place. Here are the hours of direct sun on that window before and after, with nothing else in the way:

The same block takes very different things from different windows:

• A south window loses its winter sun: more than five hours on the shortest day, almost nothing in spring and summer, because the high sun clears the roof.
• A west window keeps its short winter afternoon but loses about three hours of summer evening sun.
• A southeast window loses its spring mornings.

This is why "it is only four storeys more" tells you little. You need the height, the distance and your window's direction together.

Find the height and the position on the plans

Planning applications are public. In the UK, search your council's planning website by street or postcode. Look for:

• Elevations and sections. The drawings from the side. They show the height, often in metres above a fixed datum, or by storeys. Each storey is roughly 3 m (10 ft).
• The site plan. Where the building stands, so you can measure the distance to your window on a map.
• A daylight and sunlight report. Larger schemes often include one. It tests nearby windows and says which ones lose light. Check whether your windows are in it.

Then the quick rule: the new building blocks your sun whenever the sun is lower than the angle from your window to its roof. In London at midday on 21 December the sun is about 15° up. A roof 14.5 m above your window and 20 m away rises to about 36°, so in December the sun stays behind it for hours.

What UK councils look at

• Loss of light counts. The Planning Portal lists "loss of light or overshadowing" among the material considerations a council can weigh. Loss of view and the effect on property values do not count. The council decides what matters in each case.
• The BRE guide is the usual yardstick. Most councils use the BRE's "Site layout planning for daylight and sunlight" (2022). For sunlight it checks windows that face within 90° of due south and overlook the site. It suggests people will notice a loss when a main living room's annual probable sunlight hours fall below 25% of the year's total (5% in the winter months) and drop to less than 0.8 times what they were. For gardens, it suggests at least half the area should get two hours of sun on 21 March.
• The guide is advice, not law. In its own words it is "not mandatory", and councils can use other targets, for example in dense city centres.

Right to light is a different thing

A right to light is a private property right, not a planning rule. It usually comes from at least 20 years of uninterrupted light through a window, under the Prescription Act 1832. It protects "sufficient light according to the ordinary notions of mankind", not all the light you have now, and it is enforced through the courts, often by an injunction. Planning permission does not cancel it, and a planning objection does not enforce it. If you think you have one, talk to a right-to-light surveyor or a solicitor early.

In the US and Canada, the rules vary from city to city. Your city's planning or zoning office will tell you how to comment on an application.

Check it at your address, step by step

Flatwalk's free tool, Sun at your window, lets you add a building that is not built yet and shows what it does to each window.

1. Type your address under "See the sun at your window", and find your building on the satellite map.
2. Choose A flat ("One floor of this building") and set your floor, or A whole house.
3. Put your windows on their walls. Each wall has a tag, such as "South wall · sun side". Tap a wall, then tap where the window is, or drag to draw it.
4. Look at your sun now. Pick the Shortest day and a spring month, and read the hours and the 12 month bars. Tap Copy link to save this view.
5. Add the new building. Tap Something off?, then Add a building. Tap its corners where the new block will stand on the map, and set its height in metres from the planning drawings.
6. Read the change. The tool says "Recalculating", then shows a badge with the change, such as "-1 h 05 today". Look at the month bars again, and at "What takes your sun", which now names the new building with the months and hours it blocks.
7. Copy the link again. Links save the added building, so the two links show your sun before and after.

Putting it in your objection

Keep it short and factual:

• Which windows and rooms. For example: "Our living room window faces south-southeast, on the first floor."
• The hours before and after, on the shortest day, in March and over the year, read from the tool. Say which height you used and where it came from on the drawings.
• The two links, so the planning officer can open the same view.
• What you ask for. For example, that the applicant's daylight and sunlight report tests your windows, or that the height is reduced.

Say plainly that the hours come from a free modelling tool, not a survey. That is honest, and it keeps the officer's attention on the drawings.

What it cannot tell you

• Modelled, not measured. The hours are worked out from maps and the sun's position, not from a light meter at your window.
• Not a legal report. It is not a right-to-light survey and not a BRE daylight and sunlight assessment. It shows direct sun, not the BRE's percentages.
• The new building is only as accurate as you draw it. Use the height and position from the drawings; a flat-topped box stands in for any roof shape.
• Where the data is best. Building heights are measured in London, Manchester, Cardiff, Edinburgh, Glasgow, Berlin, Munich, Hamburg, Warsaw, Kraków, Wrocław, New York City, Chicago, Toronto, Vancouver and Montreal. Elsewhere they are estimated, and the tool says so; outside the areas it covers it is slower and treats the ground as flat.
• Other new buildings and trees. Anything built after the map data, and trees planted or cut since about 2017 to 2019, are missing or wrong until you fix them.
• Weather. Typical weather is a monthly average from 2005 to 2023, not a forecast, and can be about 10 to 15% off.
• Windows only. It does not measure a garden or a terrace floor.
• Small things it ignores: window frames, glass tint and your own roof shape. Map outlines can be a metre off the real wall.

Try it free: add the planned building and see your sun before and after

What Flatwalk does that older planners skip

Lights

Add ceiling lights, lamps and LED strips and see where you need them after the sun goes down.

Your flat in its city

Enter your address and the street and buildings around you appear in 3D, wherever the map has them.

Sun for your address

Real sunlight by date and hour, and how long each room gets direct sun.

One piece, in your own room

Through the live camera or in a photo of the room, stand a sofa on your own floor at its true size.

Detail

Window and door types, kitchens, wardrobes and bathrooms built piece by piece.

Made for a thumb

Slide the hour with one finger and watch the light move through the rooms.

Swipe for more. Every picture is the app itself.

One flat, free

Questions people ask

Will a new building block my light? It depends on its height, its distance and which way your windows face. A block to the south takes the most winter sun; a block to the west takes summer evenings. Add it to a sun tool at your address to see the hours each window would lose.

Can I object to a building blocking my sunlight? In the UK, yes: "loss of light or overshadowing" is a material planning consideration, so comment on the application before the deadline. Say which windows and rooms lose sun, and by how much.

Is loss of light a material planning consideration? Yes, the Planning Portal lists it. Loss of view and the effect on property values are not.

What is a right to light? A private right to enough light through a window, usually gained after 20 years of uninterrupted light. It is enforced through the courts, not through planning, so ask a right-to-light surveyor or solicitor.

How tall will the new building be? The elevation and section drawings in the planning application show it, in metres or storeys. Your council's planning website has them.