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Buying a house to make it MUCH BIGGER

Updated: 5 days ago



Have you been trying to buy the ideal family home but can't seem to find the right property? Maybe you’ve decided to buy a house, and then make it much bigger, so it works for you.


This house was in the right location for my client buy before they decided to buy the property, they needed to know if they could enlarge it significantly. So they reached out to me and booked a consultation. The house was too small and they had big plans

Front of the house. Copyright Chancellors.

I have been a self-employed Architect in the UK since 2009. I run the Real Life Architect, where anyone can book a consultation with me to discuss altering and extending a home or building a new house in the UK. And sometimes I write posts about these properties.


I was contacted by the client when they were looking to buy this house in July 2023. The client had four main aims for the property;


  1. Building an annex or granny flat in the rear garden.

  2. Extending the existing house to the rear garden.

  3. Converting the loft of the house to a habitable space.

  4. Subdividing the house at some future point, so the ground floor is one flat and the upper floors are a separate flat.


That is a lot to take on for one consultation, each of those aims could be a project on its own. So let's start with the annex in the rear garden. Commonly called a granny flat, the planners refer to them as ancillary accommodation and there has been steady demand for these over the past decade. As house prices have increased, younger people have found it harder to get on the property ladder. While, at the same time, the UK population is rapidly aging.

Middle-aged homeowners have been trying to solve these two problems by building self-contained accommodation in the back garden.


It's important to be aware of the phenomenon of ‘Garden Grabbing'. This is the practice of dividing up suburban gardens to create more homes and it is a controversial topic in parts of the UK. If you get planning permission to build ancillary accommodation, it usually comes with a condition stipulating the granny flat can never be sold off as a separate property. It remains forever tied to the house, it doesn’t get a separate postal address or title deed. My client was ok with this, they wanted the annex to accommodate extended family and had no intention to sell this off separately.


Now, the next three aims all need to be considered together, because they will all interact. The client needs a proper family-size home as soon as possible and to achieve that they want to extend into the garden and convert the loft with a dormer. Just like the next-door neighbours did a few years ago.

Hip roof

Initially, my client wanted to create a dormer on this hip of the roof but, after checking the local planning department guidance, I found this document.

Planning guidance for dormers to hip roofs

Long story short, the planners will only support a small dormer on a hip roof and I can't see it being big enough to contain enough space to make any real difference. So I told my client not to bother with that.


Building a dormer on the rear of the roof would be more likely to get through the planning process. The local planning department publishes guidance for rear dormers and this is more flexible.

Planning guidance for dormers to rear roofs

My client could choose between a traditional pitched roof dormer or a more spacious flat roof dormer but the hip will cause problems here too. The roof of the dormer cant extend beyond the hip without the dormer being visible from the road or getting shorter and, at a certain point, the headroom in the loft will be impacted. If we take the neighbours dormer as a guide, I expect the geometry to work out something like this.

The rear of the house. Copyright Chancellors. The maximum dormer is shown in red.

There are two other issues to consider.


First, the more bedrooms in a house, the more cars that house can have, in theory. Planners have formal guidance for this, a certain number of cars per bedroom. It doesn’t matter if my client promises to only use one car, the house could be sold or leased to others and they could use far more cars. Where this becomes a problem is if a house hasn't got enough space for those cars to be parked within its own cartilage. In that scenario, the cars will inevitably be parked on the street and that is usually a big no-no.

However, I checked the planning file for the neighbours house and, when they enlarged it, the planners consulted with the local roads department and they didn't object to more cars on that street.

Excerpt from the neighbours planning approval.

This is highly usual and it could also be misleading. You see, if my client relies on this, the policy might change by the time they make their application. Planning policies change all the time and they usually react to local political pressure. Too many cars on the street, the community complains to their councilor, who demands the road department change their policies and the planners stop allowing new developments to park cars on the street.


The other problem faced by converting the loft, is that the top floor will be more than 4.5m above the ground. This places it in the next category for fire risk and the client faces a choice. Either install sprinklers throughout the property, in every room, or install self-closing fire doors to every room off the stairs and hall. This work is further complicated by my client's desire to convert the house into two separate flats in the future. Planning for this subdivision should begin now, so when the loft is covered and the extension built, they are done in such a way that it makes the later separation easier. The kind of issues that come up are subdividing utilities, like the gas, water, electric, drainage, TV and phone. It is possible to start planning where these will enter the building and how they will get to the new upper flat.


Next to consider are things like party walls. Let's imagine the ground floor layout is changed and the current hall becomes a common hall, shared by both flats. The existing partition wall must now be classified as a separating wall and it must be much thicker, to resist sound transmission and fire.


Edited floor plan of house (original copyright Chancellors) Possible future layout. The red walls must be thicker, Separating Walls

The first floor must also be upgraded to meet the same requirements and this isn't easy. But since the client will be carrying out work inside the house to make it larger, it makes sense to do all these things now.

One final thing to consider is which flat gets to keep the granny flat. In all likelihood the original house address will remain with one of the flats and the other flat might have the same number but with the letter A attached. So No. 25 and No 25A, for example. And the original postal address keeps the granny flat.


When it comes to the extension we didn’t go into too much detail during the consultation. The main issue here will be construction costs. In the UK in 2023, the cost of basic but competent construction is about £2750 + VAT per square meter. This will apply to the granny flat as well because that must be built to the same standard as the extension. And it needs to have its own kitchen and shower room so that it can be self-contained.

Pricing internal alteration and dormers is much more difficult. You can't use a simple square meter rate. The only way to price that kind of work is to itemise each element and cost them individually. Demolition, wiring, plumbing, new internal partitions and doors, remedial work, roof work, and fitting kitchen units. Each element must be worked out individually to get an accurate price.


If you are planning to build a house extension anywhere in the UK, and you would like advice from an experienced architect, get in touch and book a consultation with me to discuss your ideas, timescale and budget. I provide a sense check on your ideas and can advise you on how to find the right local architect for your project.


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