Extension calculations

[Hoolahoophop]

Hoping to add at least 50% to the size of my property. It's 185m2 currently. Would like to get closer to 300m2 Mostly addition of large open plan kitchen, dining, living area to the ground floor and master bed with dressing and on suite to the second floor. May need a little re-jig of some rooms downstairs.

We are in the SE

Plenty of room around the property. I've never extended before, any idea where I look for information for rough estimates before I start getting architects and builders involved for quotes.

Thanks.

Before you do any of that, have you checked if you're even likely to get PP to do it? There may be local planning policies in place that limit the maximum size of extension. It refuse it on the basis of overdevelopment.

I think it should be ok, as there are several larger dethatched houses in the area and it sits centrally to its plot of a little over an acre the only issue I would see would be going up.

Your main issue will be whether you are in the Green Belt, or a designated area like an AONB or National Park - much of the south East is (I assume from the reference to a large plot you are not in London).

Also of relevance is whether the house has been previously extended.

It is within a conservation zone, edge of a small market town. Never been extended previously, built in the 1980s. The only restrictions on the conservation zone i have experienced in the past were the need to put in wooden windows rather than PVC and the trees in the garden have to stay or if unsafe be replaced from a list of acceptable species. Two doors down is a new build, twice the size of mine on a similar plot but i know they were restricted in height, as was another plot down the road up for sale with planning consent for a 1.5 story house (that one is on a hill so partially basement level). Two doors up the road was extended in the last 10 years by about 50% Conservation area has been in place for at last 25 years so these houses must have met that criteria. Any development would not impact neighbors light privacy etc as would be well within boundaries and it only really has neighbors on two sides (is down a private lane off the main street and backing onto scrub land.

There's rules of thumb that you can get from Google (£2 to £3k per sq metre rings a bell.)
However your extension is so much bigger than average I'd wonder if those rules would work, you must get some economies of scale building so big.
There are design and build firms who might come round and give you a very approx idea of costs, if they're interested in such a big project.

It will depend how it’s done. Our extension cost more than knocking down and starting again, but we wanted to retain the period facade. It was about the same 50% increase but a lot of the foot print was rebuilt. We also added a strip down one side so the whole roof needed doing. It came in at £3.2msq Finish matters too and adds cost. Best to speak to a builder

Speak to the planning officer at your local authority for a quick bit of guidance. That could give you the heads up.

Our planning office refused to give any advice. The architect and builders were much more helpful.

Yes, planners are very reluctant to comment. Say put in for permission and then we will tell you if its allowed or not! But not why its not. A friend has recently done through the same with the local planning department.

Engaging builders, architects costs money to decide if its worth spending the money. So frustrating.

Well it really will be more than you think. I’ve know lots of people start and then reduce plans once quotes are in £2-3k per sq m isn’t going to be far off

@Hoolahoophop A decent architect will know the planning policies that apply to your location. They will also know building costs. That depends on quality of build, complexity, finishes etc. Plus it will always be way more than you think. Then you need to think about the garden around the extension, matching bricks, roof etc.

Around here we have restrictions on size of extension based on size of house in 1948! Therefore a new house can be 150% only of that volume! Many people build new to reset the dial. They then apply for an extension on the new house. The big problem with extending is making sure the two halves look like one house with building materials and balanced elevations.

Well two years ago our 50m2 extension including internal alteration, new kitchen and bathroom, plus a new roof to the main house was £220k so I predict yours will be at least double that.

I would suggest engaging with an architectural technician as they will be cheaper than an architect and probably better placed to advise technically etc. we paid around £2k for survey, existing drawings, planning drawings and application and building regs drawings.

Technicians are not better placed “technically”. If you need structural calculations, you need a structural engineer. Architects are trained to look at how a living space works. They should have invaluable advice. I would also consider building completely new. That will get you what you want.

Definitely can't afford to knock down the house and start again. And don't fancy caravan. We love our location....just want more space.